It means that the job is only for a few hours a day,or a few days a week. A full time job would be for example 9 am till 5pm every day.A part time job might be only mornings,or evenings,or maybe only two or three days a week.

To add to Ruthi's explanation, a full-time job is considered 40 hours a week. It also might include certain benefits like healthcare and pension. A part-time job is less than 40 hours a week. it is often paid hourly.

A part-time job is definitely not for a whole day and salary sure lower than a whole-day work also means a full-time work.