In skateboarding and snowboarding it is two things. The first is easy to explain, it is simply a grab. The second is much harder to explain, but it is basically an extension of a grab, similiar to an invert.

The Grab: This is a grab of intermediate difficulty. Learn how to do the mute first. Once you can do that, bend your knees and then do a mute with your knees bent as such. Next push one knee down and the other up. The board will "tweak". If you want more style/points, then all you need to do is "tweak" in the other direction. When you are landing, straighten the board and return your knees to their normal position.
Note: In skateboarding you will have to grab the board before bending your knees or you will land without the assistance of your trusty board.

The other thing: This is taking the "tweak" part from the tweak grab and lending it to almost any other grab. You can tweak indys, mutes, stalefish... almost any grab. This is different to a bone.