What does "look out for" mean?

See to the welfare of, as in Mary was assigned to look out for the youngsters on the playground. Similar to look after, this expression appears in such terms as look out for number one, meaning “see to one's own best interests,” as in Looking out for number one is Barbara's first priority. Versions of this expression, such as take care of number one, date from 1700.

Be careful of or watchful for something or someone, as in Look out for broken glass on the floor, or Look out for Mary—she'll be coming any minute. [Second half of 1600s] Also see look out.