Tadaima means "I am home" or "I'm back." You say this when you return home, addressing the people inside. It's a common and expected greeting upon entering your residence. It signals your return. For example, when you walk through the door, you'd say, "ただいま！" (I'm home!).

家(いえ)に帰(かえ)ってきた時(とき)に、中(なか)にいる人(ひと)に言(い)う言葉(ことば)です。自分(じぶん)の帰宅(きたく)を伝(つた)えます。「ただいま！」