The propinquity effect is when people are more likely to become friends with or form relationships with people who are physically close to them. This means that if you are around someone a lot, like a classmate or neighbor, you are more likely to become friends with them than someone who you only see once in a while.
It's kind of like when you have a favorite toy that you like to play with a lot. The more you play with it, the more you like it and want to keep playing with it. The same thing can happen with people - the more we are around someone, the more we like them and want to spend time with them.
This doesn't mean that you can't become friends with someone who isn't physically close, but the propinquity effect just means that it's more likely to happen with people who are nearby. So next time you're in a new class or neighborhood, try to make friends with the people who are closest to you first!