Generally, it means at least 1,560 hours of paid work (about one year full-time at 30 hours per week, or the same total hours part-time) in a skilled occupation (TEER 0, 1, 2 or 3). The work must be paid; volunteering and unpaid internships don't count. Your duties must match the NOC you claim, and you'll need documents like reference letters to prove the hours, pay and responsibilities.