People already explained that's it's how much supply you have.

In practice this is easier for people to use than actual clock timings, because it's more robust to delays or interference. If you remember "third rax at 30 supply" then even if you're playing a little slow, you will still know roughly when to build that. But if you memorized exact clock timings and now you might be 20+ seconds behind, it's hard to know when you should fit in the new building.

It's not perfect of course, and if you get cheesed and the game goes weird then you'll have to start improvising rather than relying on just supply timings, a lot of times after a cheese where neither side definitely wins, the balance between tech and economy is now very non-standard and you can't rely on conventional rules of thumb anymore.