Transactionalism is like when you trade something with someone else. Imagine you have a toy and your friend likes it a lot. You might say, "I'll give you my toy if you give me your treat." That's a transaction - you're trading one thing for another.
In grown-up terms, transactionalism is when people or groups interact with each other in a way that benefits both parties. So instead of just trading toys and treats, it might be money for goods or services, or giving up something you want in order to get something else you want.
Transactionalism is important because it helps people and groups meet their needs and goals. If you have something someone else wants, you can use that to your advantage to get something you want from them. It's like a big puzzle where everyone has a piece that someone else needs, and by trading and working together, everyone can get what they want.