Tying in commerce is when a company makes you buy something you don't really want in order to get something you do want. It's like having to eat broccoli (which you don't really like) in order to get a cookie (which you love).
For example, let's say you want to buy a new computer. But the company that makes the computer says you have to also buy their expensive printer in order to get a discount on the computer. You might not need or want the printer, but you have to buy it anyway to get the discount.
This can be a problem because it limits your choices and forces you to spend more money than you want to. It's like the bully on the playground who only lets you have a toy if you trade him for something else he wants.
Tying can also be bad for competition, because it can make it hard for other companies to sell their products. Imagine if all the computer companies tied their printers to their computers - it would be hard for other printer companies to sell their products, even if they were better or cheaper.
In summary, tying is when a company makes you buy something you don't want in order to get something you do want, which can limit your choices and hurt competition.