Roll your "r"s

What is a rolled "r"? I keep reading about this but I don't know what it is. Can someone explain or give an example?

I learned how to roll my R's from Japanese. I can keep my R rolling for a good 30 seconds, but I think the Japanese roll might be different? A russian person told me mine sounded too Japanese if that's possible. Basically, if you're doing R and expecting it to roll, it won't happen. The roll is like with the tongue movement of an L while trying to say R (which is why Japanese get L and R mixed up so easily, but I don't see any other way possible)
Basically your tongue tip will touch the roof of your mouth, the air to make the sound will push this tip but the pressure from your tongue will kind of make it fall back, causing the roll. Obviously, if you put too much pressure just air isn't going to push it, so it's got to be borderline touching.
Honestly, the easiest word to practice is probably with 'drill' as the D sets you up in the position like an L, and you make the R sound right after to get that roll