Hitting the target for sure; certainty of striking a mark. By extension, used to mean a sure success or a guaranteed hit (noun; often used with 'suru').
その選手の矢は必中で、的を外すことがない。
That archer's arrows are sure to hit; he never misses the target.
この作戦は慎重に練れば必中だという自信がある。
I'm confident that if we plan this strategy carefully, it will surely succeed.
必中の策などこの世にない、と彼は言った。
He said there is no such thing as a guaranteed plan.