Secondary liability is like when a grown-up is responsible for something a kid does. For example, if you and your friend are playing ball in the house and your friend breaks a vase, your parents might say they are responsible for paying for the vase because they should have been watching you and your friend.
In the grown-up world, secondary liability means that if someone helps someone else do something wrong, they can also be in trouble. For example, if you and your friend want to cheat on a test and your friend asks you to help them by giving them the answers, you could get in trouble too for helping them cheat.
Another example is if someone makes a fake website to sell illegal things, like fake medicine, and they use a website hosting company to put the website on the internet. If the website owners find out the website is illegal and they don't take it down, they could also get in trouble for helping the person who made the website.
So basically, secondary liability means that even if you didn't do anything wrong yourself, you could still get in trouble if you helped someone else do something wrong.