「立つ」 (tatsu) means "to stand." It describes the action of moving from a sitting or lying position to an upright posture. It's an intransitive verb, often used to instruct someone to stand up or to describe someone being in a standing position.

「立(た)つ」は、座(すわ)った状態(じょうたい)から体(からだ)を起(お)こし、まっすぐな姿勢(しせい)になる動詞(どうし)です。