I am making a Firbolg Druid, and I was wondering if the racial feature Hidden Step (Invisible as a Bonus Action) counts as a Spell or a Cantrip? I can’t find anything that specifically says. I want to possibly use a spell and then use the Hidden Step and move away.
It does not count as either a spell or a cantrip.
An ability is a spell if it says it counts as a spell, or if it references a specific spell in the book.
For example, the Drow's Drow Magic ability says:
You know the dancing lights cantrip. When you reach 3rd level, you can cast the faerie fire spell once per day...
That hits on both points: those are specific spells from the book referenced by name, and the description uses the words 'cantrip' and 'spell' just to be extra clear that they are in fact those things. So you can't, for example, use your racial faerie fire if you already used a bonus action to cast healing word this turn.
But the firbolg's Hidden Step doesn't say any of that, so it isn't a spell or a cantrip, and doesn't restrict your ability to cast other spells during the same turn. It's just a special bonus action you can do, like a rogue's Cunning Action.
The ability states that you magically become invisible. You do not have to cast the Invisibility spell to do so.
Feats and spell-like abilities are not spells
Jeremy Crawford, lead rules designer for D&D 5e, says:
Want to know what spells look like? See "Spells" (PH, 211–89). Nothing else is a spell unless it is presented as a spell or called one. (Crawford)
See this answer on a related question for a detailed discussion of what is a spell.
Since a racial feature is not a spell, there is nothing preventing you from casting a spell and then using Hidden Step.
As Destruktor pointed out, Hidden Step is not a spell or cantrip, but a racial ability. A feature is only a spell or cantrip if there is text in the rules saying it is.
That being said, I think you're misunderstanding the rules on bonus action spells, since you said that the reason you wanted to know if Hidden Step was a spell or cantrip was:
I want to possibly use a spell and then use the Hidden Step and move away.
Note that cantrips are examples of spells, and the rules say that:
A spell cast with a bonus action is especially swift. You must use a bonus action on your turn to cast the spell, provided that you haven’t already taken a bonus action this turn. You can’t cast another spell during the same turn, except for a cantrip with a casting time of 1 action. (PHB., p. 203)
So if Hidden Step had been a cantrip that took a bonus action to cast (like shillelagh is), you also couldn't use your action to cast another spell (other than another cantrip). So it's a good thing that Hidden Step is not a spell (cantrip or otherwise).