This is a term being thrown around in the Smash Bros. community. Apparently it's a move that you do, but the manual doesn't mention anything like it. What does it mean?
L-Canceling via Smash Bros Wiki. L-canceling does not work in Brawl, although auto-cancel does.
To L-cancel, press L, R, or Z (only Z and R work in SSB) within 6 frames of landing on the ground while the character is still performing an aerial attack. If performed correctly, the character's recovery animation will be played at twice its normal speed in SSBM, effectively cutting recovery time in half. If the number of frames of the lag divided by two ends with a decimal (i.e. 15/2 = 7.5), it will be rounded downwards. In SSB, however, the recovery animation is erased completely, allowing one to move immediately after any aerial attack. L-canceling can only be used with aerial attacks involving the A button; it is impossible to L-cancel a special move. It is also impossible to L-cancel Mr. Game & Watch's neutral aerial and up aerial attack.
It is a means of recovering faster from hitting the ground while performing an air-based attack. You hit either Left, Right or the Z button within ~1/10th of a second (6 frames) after hitting the ground and your recovery will be twice as fast in Melee, allowing you to move onto your next move will less opportunity for your opponents to gain advantage.
This only works with basic air attacks, though, not special attacks.
Normally, when you land during an aerial attack, you go through an extended landing animation that aware opponents can punish you on. Lag-cancelling is a technique where you press a shield button 6 frames before you land. Doing this will reduce the landing lag time to 4 frames in SSB64, while it cuts the landing lag time in half in Melee (this will end up being rounded downwards because of how subframes work). L-cancelling has no effect if you land during the autocancel window of an attack (the time at the start and end where you don't get the extra lag from landing), and some attacks cannot be L-cancelled: notably all special attacks plus Mr. Game & Watch's neutral, back, and up aerials (since they use nonstandard states for some reason).
Since L-cancelling has almost no downside (it causes hitboxes produced on landing to not last as long), you want to be using it every time; characters like Bowser and Ganondorf have their slow aerials turned into quickly-ending attacks that can be followed up on easier, and the SHFFL (short hop fast fall L-cancel) allows any (non-floaty) character to throw out a very fast string of aerial attacks at ground level. For Melee specifically (since the question is tagged as such), you may want to be pressing Z instead of L or R simply because it's easier to press in an accurate window (because Z is both an attack and a shield button, it works for L-cancelling).
Some people call the technique Z-cancelling in SSB64, and the Japanese site translates it to "smooth landing".