Concentration
Keeping your focus to hold a spell going; getting hurt can break it and end the spell.
Some spells only last while the caster concentrates on them. Casting another such spell ends the first, and taking damage may force a roll to keep concentrating. Whether and how this applies depends on the ruleset.
In Fantasy RPG
Fantasy RPG uses concentration. A caster hurt while concentrating makes a Constitution save, DC 10 or half the damage taken, whichever is higher, or the spell ends.