Casual work is when someone works a job that is not permanent or regular. It is like when you go to your friend's house to play and your friend's mom asks you to help her clean up the toys. You are helping out, but you do not always do it and you do not get paid for it every time.
In casual work, the hours and days you work can change all the time. It is like when your mom asks you to clean your room on Saturday, but sometimes she will ask you to do it on Sunday or maybe next week.
Usually, people who have casual work do not get the same benefits as people who work full-time. Benefits are things like vacation time or health insurance.
Casual work can be good for people who want to work but also need to keep their schedules flexible. It is like when you want to play outside with your friends, but sometimes you have to go to the store with your mom. You can still have fun with your friends later, but you need to be flexible with your plans.