Grapple canceling is a technique available to Samus, Link, Toon Link, and Lucas (the characters who's grab is a grapple) in Super Smash Bros. Brawl. The basis of it is the fact that midair grapples are immediately canceled with no lag upon landing.
The easiest way to use the grapple cancel is to short hop. On the way down from the short-hop, press Z (or the equivalent grab button on a non-Gamecube Controller). If the character has a midair grapple move, it will extend quickly, and lightly damage those who touch it. Upon landing, there will be no lag, and any other move can be performed immediately.
This is very useful with Samus, as her midair grapple (also known as a Z aerial or Zair) comes out quickly, has a long range, and has decent stun time on it. Samus can even short hop a Homing Missile and start her Zair upon completion of the Homing Missile, canceling when it hits the ground. This technique is great for pressuring, approaching, and stopping approaches.