Okay, so imagine you are playing with toys with your friend. You have your own toys but sometimes you share and play together. It's a nice way to have fun, right?
Now, let's imagine that you and your friend live in different houses but they are very close to each other. Sometimes, when you play together, you cross the imaginary line between your two houses without even noticing - you are now in your friend's yard!
Well, the Welsh Marches are a bit like that. Hundreds of years ago, there were two big kingdoms in the United Kingdom: England and Wales. They were different in many ways, including language and culture.
But sometimes, people from England would cross the border and go to Wales, while people from Wales would cross the border and go to England. They didn't really think too much about it, it was like a big playground for them.
Over time, some parts of the border became more important than others. Those were the areas where people from both sides would often go to trade, or to meet their friends, or even to fight! Those areas became known as the Welsh Marches.
So to sum up, the Welsh Marches are the areas along the border between England and Wales where people from both sides used to play, trade, and sometimes fight a long time ago.