Scorch is a neutral special card in Gwent. When played, it immediately kills the strongest card(s) on the battlefield, which may include your own. This is achieved by sending "killed" cards to the discard pile. Scorch can be used to powerfully alter the tide of battle, since an opponent may have multiple cards of the same strength that could be destroyed.
Playing relatively low-strength cards at the beginning of a round could be a good way to protect one's own units from an opponent's Scorch card. Otherwise, Heroes are completely immune to Scorch, as they are to all special cards, and can thus be safely played.
Scorch may be particularly useful against chains of cards utilizing the strength bonus from the Tight Bond special ability (e.g. Catapults). These cards are likely to be powerful when placed side-by-side, and would have the same unit strength, making them ideal candidates for Scorch.