ADS in sniping

Though the necessity varies between generations and games, the general idea is that, especially late in the game, you cannot reach certain areas without some of the HM moves: i.e., strength, cut, surf, whirlpool, dive, etc. (Again, the actual requirements vary by game).

An HM slave, then, is a term for a pokemon that is not used for battle but knows one or more of these moves, such that the battle performance of your other 5 pokemon is not diminished by "wasting" one of your 4 attack moves with an HM move (which are generally subpar compared to other, similar, options).

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.