敵てきを知しり己おのれを知しれば百ひゃく戦せん危あやうからず

also: てきをしりおのれをしらばひゃくせんあやうからず

Meanings

know your enemy and know yourself, and you will not fear a hundred battles

Proverb derived from Sun Tzu's Art of War. Emphasizes that thorough knowledge of both the opponent and oneself ensures victory.