I just started playing D&D 5e and it's the first time I've played D&D or anything like it. I've seen people referencing "PHB" but I don't know what that is.
Get Ready for Some Letters
It's the "Player's Handbook:" the book every player needs to start playing. It details most of the rules, classes, and races that people can be when they are a player. It's also helpful for Dungeon Masters ("DM"'s), but DMs usually like having the Monster Manual ("MM") as well. The MM lets DMs have some pre-made monsters and traps to throw at their players. There is also the Dungeon Master's Guide ("DMG"), which helps them be better at making and running adventures with players.
You can find a free, but only the "basics" version of the PHB here, but it does not have as much stuff as a the full price book, like more class options and feats.
I personally suggest you buy one PHB and share it with the group you're playing with, or buy it for yourself and bring it to adventure league nights at your local gaming store.
This list on Reddit can help you clarify most of the acronyms that float around 5e discussion.
In this case, PHB refers to the Player's Handbook, the basic rules and guidelines for playing 5th edition D&D.