Pink collar refers to jobs that are traditionally held by women such as nursing, teaching, and administrative work.
Imagine you have a bunch of crayons and some are blue and some are pink. The pink ones are usually associated with girls and the blue ones with boys. When it comes to jobs, certain ones are often thought of as "girly" like being a teacher, while other jobs like being a construction worker are thought of as "boyish." The jobs that are considered girly are the ones that fall under the pink collar category.
So just like how pink is seen as a girl's color, pink collar jobs are typically held by women. However, this doesn't mean that men can't do these jobs too! It's just that over time, certain jobs have become associated with certain genders.