L-canceling

L-canceling is a technique in both SSB and SSBM for reducing the length of recovery animations after landing on the ground while performing an aerial attack. By pressing Z, L, or R slightly before landing on the ground, a player can reduce their recovery time, a significant decrease that greatly increases a character's maximum rate of attack.

Technical details
To l-cancel, press L, R, or Z (only Z and R work in SSB) within 20 frames of landing on the ground while your character is performing an aerial attack. If performed correctly, your character's recovery animation (example: Link pulling his sword out of the ground after a down-aerial) will be played at twice its normal speed in SSBM, effectively cutting your 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-cancelling can only be used with aerial attacks involving the A button; it is impossible to l-cancel a B move. It is also impossible to l-cancel Mr. Game & Watch's neutral aerial and up aerial attack.

Auto-cancel
Auto-canceling occurs when the character lands at the earliest or last frames of an aerial attack. This causes the character to land with normal landing lag (usually 4 frames) without any additional lag from using the attack. When an attack is auto-canceled, L-canceling has no effect.