Okay kiddo, so imagine you have a toybox and you like to share your toys with your friends. Let's say you have three friends and three toys. You want to make sure everyone gets to play with a toy, so you decide to give each friend a toy for a certain amount of time.
Supply management in Canada is kinda like that toybox. Except the toys are things like milk, eggs, and poultry (like chicken). There are also a limited number of people who produce these things (like your three friends). The government wants to make sure that everyone who wants to buy Canadian milk, eggs, and poultry can get it, but they also want to make sure that the people who produce these things get paid enough to keep producing them.
So the government came up with a plan to limit the amount of milk, eggs, and poultry that can be produced in Canada. That way, there's not too much of it, and the people who produce it can make a good living. The plan is called supply management.
Basically, the government sets a limit on how much of these things can be produced in Canada. The people who want to produce them have to buy a permit from the government to do so. This makes it more expensive to become a milk, egg, or poultry farmer, but it also guarantees that the prices for these things will stay stable and not go up and down too much.
So even though it might seem weird to limit how much of something can be made, supply management helps make sure that everyone can get what they need (like you and your friends sharing toys), while also making sure that the people who work hard to produce these things get rewarded fairly (like your friends who played with the toys getting their turn to share with you).