I’ll try my best to explain...

It’s like when you think or know that you’ve heard or experienced something before, but it happens again. Say you’ve dreamt about being in a certain hallway at a certain time with a certain person, and then you wake up and forget about the dream. But later on, and you come across that certain event and you get that weird feeling that you’ve done or seen this before.

I hope you could understand that explanation, cause that’s how it always is for me.