If you think of your Google Account Storage like a real world storage unit, these concepts will be easier. Let’s say you had a small storage unit that would hold 15 boxes of stuff. Once it gets full, what happens? You can’t store any more stuff! If you take out some of the old boxes, you can put in new ones, but you’re limited to 15 boxes total.
3 Choices when your storage is full
It’s the same with your Google Account Storage. Google gives everyone 15GB of storage for free. Once you fill that up, you have 3 choices: