Splinter is a jumpshift showing you are short in the bid suit and has support to partners suit. Often the trumpsuit is agreed to before the splinterbid.
Splinter is used in many positions. Originally it was used after 1NT opening were 3D/H/S Over 1NT: shows a 3-suited hand with a singleton or void in the suit bid. Forcing to game.
You can also use a double jumpshift after opening on the 1-level as short suit and strength enough to game.