Edgeguarding (also called edge-guarding) is the act of attempting to stop your opponent from successfully recovering. The opposite of edge-guarding is guard breaking.
Types of Edgeguarding
These guards involve spamming a projectile to push an opponent gradually off the stage. This type of guard is easily broken by recovering from below, but can be lethal against a character who has a below-average vertical recovery. For these characters, air dodging at the right time is vital, or they will get KO'ed. This can be done with Link's standard and down attack, Young Link's standard and down attack, Mario's standard attack, Luigi's standard attack, Knuckles' down attack, Samus's standard, side, up and down aerial attack, Fox's standard attack, Mega Man X's standard, side, down, up and down aerial attack, Zero's standard attack, Mewtwo's standard and down attack, Lloyd's down attack, Sheik's standard attack, Tails's standard, up and down aerial attack and Mr. Game and Watch's down attack.
These guards involve the edgeguarder jumping slightly or completely off the stage to deal an aerial attack to a recovering opponent. This puts the opponent in a very tricky situation, forcing them to mix up their recovery options to guard break it; however, in cases where a character's recovery is linear, predictable, and unsafe, they usually cannot deal with this type of pressure, making it extremely efficient against these types of characters. If you're not playing as Meta Knight, Kirby, Jigglypuff or Tails, it is best to jump once to attack that opponent and then use your second jump to come back. If not, it is likely you would fall and get self-destruct.
Wall/Fence of Pain
This is a guard in which the edgeguarder repeatedly attacks the guarded with a repeated amount of attacks to knock him or her towards the blast line for a KO. It is best performed with Jigglypuff, since her attacks don't give enemies the time to react out of them, but a "Fence of Pain" can be performed by Kirby, Tails, and Meta Knight. Kirby's Fence of Pain can be done with his side, down and up attack, Tails's can be done with his side attack, and Meta Knight's can be done with his side attack.