Sometimes means ‘not always or not often’:
We usually played football at school, but sometimes we played hockey.
I sometimes wonder whether I should give up my job and just travel.
Sometime means ‘at an unknown or undecided time in the future or the past’:
Why don’t you come over and have dinner with us sometime? (at an undecided time in the future)
I knew I had met her sometime, but I couldn’t remember when.