So, you know how grown-ups go to work every day? They have to wake up early and leave the house to go to their job. 9 to 5 is the time that most grown-ups have to be at work.
From 9am in the morning until 5pm in the afternoon, they have to do their job and work really hard. This is called a "work day".
It's just like how you have to go to school every weekday from 9 in the morning until 3 in the afternoon. But instead of going to school, grown-ups go to work.
When their workday ends at 5pm, they can come back home to spend time with their family and friends, just like how you come back home from school. And then they have to do it all over again the next day.