On the Beacon's Playbook sheet, it lists some potential abilities for the Beacon. One of them says "phasing". What does this refer to? My reading of it seems to be that it is a superpower, but isn't the Beacon supposed to not have any superpowers?
You determine what the Phasing ability means for your Beacon as you create them, and determine or discover more during play. (Or you can wholly defer finding out to during play, and decide nothing about it during character creation.) It's deliberately left undefined to enable you as the Beacon's player to choose what it means. I imagine for most people it would sound like the ability to move through solid objects, and that's a valid interpretation. It may also be valid to interpret it as astral psychic power or whatever else resonates for you in that word.
From Chapter 3: The Heroes, page 41, on Abilities:
Your abilities leave some information up to interpretation. A Nova might have sorcery, but that doesn’t tell you exactly what sorcery looks like, how it works, or what you can do with it.
(They then present a couple of other examples, and note that in two different games, the same playbook might be chosen with the exact same abilities, but look completely different between the two games. Anyway, let's continue this section.)
Abilities don’t clearly state exactly what you can or can’t do: after all, if your ability is super speed, that doesn’t tell you at all whether you can travel at 600 miles per hour, or break the sound barrier, or vibrate your molecules through a wall. Abilities give you a clearer picture of your character at the start of play and a jumping off point for figuring out what makes sense for your character’s specific capabilities. [...]
You’ll discover the specifics of your abilities through play at your table, through discussions with the GM, and through your own decisions. After all, you’re playing to find out!
There's a bunch that was skipped in there, but I figure with those parts you'll begin to get the idea: they're words, and you decide what those words really mean.
but isn't the Beacon supposed to not have any superpowers?
Whilst the Beacon strongly echoes superpower-less characters like Robin, he's not necessarily superpowerless. There isn't a trait anywhere on his sheet that says "has no superpowers", just not very potent ones, and if you pick phasing and define it to behave as a superpower, he definitely has superpowers. The Beacon is out of his depth and normal enough to live an ordinary life, and the other characters are on a different level to him, but he's not without access to superpowers.
Personally, last time I played Beacon, I decided he was a mostly ordinary teenage kid who thought the ninja aesthetic was super cool. I could see myself giving him "phasing" and saying it describes an unnerving knack he has to appear somewhere else unnoticed, without anyone realising he'd left his original spot, even if they thought they were paying attention. He'd call it phasing because it sounds cool like that.
But another Beacon could pick "acrobatics" and "phasing" and describe both being superpowered: he has superhuman acrobatic abilities, and he can phase through walls, or whatever else they want to choose for that to mean. It's up to them. Whatever they can do, they're still out of their depth as a superhero compared to the rest of the team.
In Masks, superpowers and abilities are purposefully left vague and open-ended so as to allow the player freedom to customize powers to the specific character they are playing. Per page 41 of the core rules:
Your abilities leave some information up to interpretation.
One valid interpretation of phasing does indeed refer to the superpower that lets one pass through solid objects.
On that same Playbook sheet it says:
If you have superpowers, they’re pretty minor or not noticeable.
For the Beacon, the only thing phasing (in this interpretation) lets you do really is run away, avoid danger, and possibly stealth/infiltration. None of these things are front-liner superpower material and pale in comparison to basically any other superpowers in the game.
It is important to remember that the Playbooks aren't there to limit the powers a hero can get as much as they are to define the main conflicts and drives for that character. As it says for the Beacon in the core rules p. 93:
You can expect NPCs to tell you that you don’t really belong here. After all, you don’t have those extras that most people associate with superheroes. A central conflict for the Beacon is proving yourself and making the case for why you actually belong.
A Beacon is all about being excited and really pumped about being a hero despite the fact that they are overpowered by those around them in terms of traditional superhero powers.
It is worth noting that phasing being one of the defaults for the Beacon is possibly a reference to Kitty Pryde from her early appearances in X-Men (despite the fact that she is not listed in the Inspirations section of the description for the Beacon). At that time she would certainly qualify as a Beacon-type character and she had phasing powers. You can see further discussion and speculation on that point here. If you are looking to play a Beacon with phasing powers as described above, this could be a good place to look for inspiration.