What is dissociation and what causes it?

It has numerous causes,  most being some sort of interference with the brain, either by substance or physical trauma.

You can look up the definition,  so I am going to just tell you what it feels like...as if your perceptions are messed up,  causing 5min to feel like a day or a certain picture or object triggering extreme deja vue as memory. 

It's experiencing the present as if it were the future and the past at the same time. 

But that was my version of it;)