The adverb sometime means "at a time that's not known or specified." So if you promise to take your friend out for pizza sometime, you're being vague about exactly when it's going to happen.
When you tell your parents you got home sometime after ten, you're either not sure exactly what time it was or you're avoiding telling them you didn't get in until midnight. Informally, you can also use sometime to mean "occasional." So if you appear in your school's yearly musical, you can describe yourself as a sometime singer and dancer.