I Don't Understand Scab-Clan Charger's Ability
An attacking creature is one that was declared as an attacker earlier in the current combat phase, or a creature placed "on the battlefield attacking" in the current combat phase (e.g. Geist of Saint Traft's angel).

Say you attack with a 3/3 and it's blocked by a 4/4. You could discard a Scab-Clan Charger from your hand to make your 3/3 a 5/7 until the end of the turn. It would survive, and the 4/4 would die (instead of the other way around).