Imagine you're building a fort out of blocks. You've placed two blocks standing up next to each other, but you want to make the top look fancy.
A gothic arch is like a fancy shape you can make with your blocks to create a special look. Instead of just placing a block straight across the top, you cut it into a point in the center and curve the sides a bit.
This makes the arch look really cool and almost like a spiderweb or an upside-down rainbow. People started using gothic arches a long time ago when building big, beautiful buildings, like churches or castles, to make them look really special and important.