In action or role playing games, 会心【かいしん】 means クリティカル/critical hit, which happens randomly and deals more damage than usual. If it appears in stats (like 会心 15
), it must be short for 会心率 (率【りつ】 = chance, rate).
Unlike "critical", 会心 is always used for good damages inflicted on enemies by allies.
However, 会心, or more specifically 会心の一撃, is strongly associated with Dragon Quest franchise, so other games use it less frequently. The safer replacement is クリティカル. ("Critical damage" inflicted by enemies is called 痛恨【つうこん】の一撃 in Dragon Quest).
Since Dragon Quest is very popular in Japan, both 会心の一撃 and 痛恨の一撃 are frequently and jokingly used in daily conversations.