A shield grab is the act of grabbing while shielding. Generally, it is used to punish attacks when they are blocked, and is the most basic way to punish out of shield; in most cases, the attacker will be caught in the lag of their attack, which means the defender's grab can connect with little trouble. Using a shield grab allows a character to circumvent the slight delay that usually occurs after dropping the shield, so unless they have a better out of shield option, a shield grab is usually the best response.
Because grabbing input-wise is the same as shielding and attacking at the same time, shield grabs can be done simply by pressing the attack button while shielding, instead of pressing the grab button.
As a basic, simple technique, however, there are many options against shield grabbing. Cross-ups and SHFFLs (SHFFs from Brawl onward) are the most effective, as they allow the player to attack the shielding opponent and position themselves either behind them, or at a distance away from them that exceeds their grab range, thus rendering a shield grab ineffective and allowing the player to punish the opponent if they attempt one. In these cases, other options out of shield that can hit behind the attacker or farther in front of them, such as short hopped aerial attacks, are more preferred. However, shield grabs are still effective when used in the right situations, mainly against moves with high ending lag that leave the attacker close to the shielding opponent, such as dash attacks and most approaching attacks. Several characters' grabs are also notoriously rewarding, with examples being the Ice Climbers' in Melee and Brawl, and Luigi's in Smash 4, making shield grabs their most potent form of punishment out of shield.
In the original Super Smash Bros., if a player uses a shield grab after experiencing shieldstun from an attack, subsequent throws will only deal half damage. It is believed this was implemented to combat shield grabbing as the "obvious choice of action". This restriction was removed in later games. However, in Super Smash Bros. Ultimate, another feature similarly weakens post-shieldstun shield grabs by forcing characters to shield for 4 additional frames before they are executed, effectively increasing their startup. Combined with the generally reduced landing lag, the increased shieldstun for non-aerial attacks, and other out of shield options not possessing this delay, shield grabs in Ultimate are overall harder to use effectively in competitive play.