It means you have this feeling of familiarity when you are at a certain place, doing a certain thing that makes you have a sudden feel that you've been to the place, or done this thing before previously. I've experienced this a couple of times in my life already, but I'm gonna type this out here is because what I've experienced recently was really really something very surprising.

I went for my class BBQ last Saturday at Pasir Ris park and when I reached there, I suddenly felt that it's like an event that I've been to before in my life. The feeling was so vivid like it's been only taken place recently. And I thought that it might just be some random feeling that came onto my mind as I've been to so many BBQs recently. But when I've reached home, I switched on my TV, tuned around channels looking for shows, and finally settled down on a variety show. So I made myself a cup of hot chocolate drink and started munching on the egg tarts my bro bought, and this time, the feeling of Deja Vu happened once again. I actually sat still on my sofa, staring into the space and trying hard to recall did this scenario happened before. This was the first time that I actually couldn't understand how did this actually took place, and how do the theory of science explain this.