A shield stab, also called a shield poke, is the event of finding a damageable portion of a character slightly outside of their shield and striking it, therefore bypassing their attempt to shield. This is an extremely useful technique, as it can be used to strike an otherwise defended enemy.
Some characters are easier to shield stab than others. For example, Mr. Game & Watch's shield is small compared to his body size (in Melee), and Captain Falcon and Pit are too tall for a spherical bubble to cover properly. A character that is short and/or round (such as Kirby or Olimar) can be hard to shield stab for similar reasons.
If a character sees an incoming shield stab attempt, they may be able to shift their shield to cover up the area by slightly tilting the control stick (tilting too far too quickly will cause a roll or sidestep). This gets harder as their shield weakens and shrinks.
Some moves are more suited for shield stabbing than others. Multi-hit moves (such as Mach Tornado) that are used at their maximum range can deplete a shield with the first few hits and then strike on the next few. The hitbox shape of a move also affects its shield stabbing viability; Marth's down tilt only hits very low to the ground, so it can hit an opponent's uncovered feet without accidentally touching their shield's center. Connecting a move with significant shield damage will render the user's next move likely to shield stab, though the target may just choose to drop their shield if it gets that low.
While shield stabbing can be useful, in some cases it can actually be a problem if the attacker is trying to get a shield break; in fact breaking a shield and leaving the target open for a fully-charged smash or super-powered special move might be preferred over hitting a shielding opponent with a basic attack. Since shields can get rather small before they break, this means attackers trying to get a break need very good aim if they want to hit the shield as opposed to the target.
Due to Yoshi's unique egg shield, which does not shrink and always covers his entire body, he cannot normally be shield stabbed. However, in Brawl, the very bottoms of Yoshi's feet hang out the bottom of his egg, which can be shield stabbed from below with very precise distancing.