お邪魔します (Ojama shimasu) means "Excuse me for disturbing you" or "I will be intruding." You say this politely when entering someone's home or office, acknowledging that you are temporarily intruding on their space. You can also say it when leaving.

人の家を訪問する際や、他人の邪魔をする時に言う挨拶です。入る時と出る時に使います。