「午後」 (gogo) means "p.m." or "afternoon." It specifies the time from noon until midnight. Similar to 午前, it's often used with numbers to indicate an exact time. For example, 「午後３時に会議があります。」(Gogo sanji ni kaigi ga arimasu.) means "There is a meeting at 3 p.m."

午後は正午から深夜まで。午後3時に会議があります。