A frameboost is a glitch where you stop grinding a rail on the same frame that you land on the rail. The result is that you leave the rail with a much higher speed than normal.
The way it works is that, when you land on a rail, the game gives you a significant speed boost (this is why hopping on rails repeatedly makes you accelerate). Then on the next frame, the game caps your speed to the normal grind speed maximum. If you jump off of a rail on the same frame you land on it, the game never has a chance to cap your speed and you get to keep the speed boost.
This mechanic also applies to falling off of rails, as well as wallrides.
Your speed is boosted by 114% every time you frameboost - by performing several frameboosts in a row, you can achieve game-breaking speeds.
Kiss the Rail
When you achieve a frameboost by falling onto a rail at the last grindable frame. You then leave the rail on the same frame, resulting in a frameboost without having to jump. Faster than a frameboost because a trick jump is not required (which reduces speed by 20%).
Kiss the Wall
Same as a Kiss the Rail, but with a wallride.
N2R (Next to Rail)
This refers to frameboosts that are specifically done by being in front or beside something rideable. When next to the rail or billboard, pressing A twice in rapid succession (once to jump into the grind/wallride, then again to jump off) will, with proper timing, cause a frameboost.
Term used to describe when you achieve a frameboost but aren't going the max air speed that you could be going, given where you jumped from. This happens due to you slowing down in the air or not being maximum speed from the point that you jumped at. Mechanically identical to a frameboost.