I don't understand this move

I’m on a 9x9 board I captured the opponent’s in the top left corner and then his chip reappears in the center…I capture that then later as I’m dominating he puts that chip back in that spot and takes all of mine at once

Go is a surrounding game. The player who surrounds the most empty space is the winner.

In your game you surrounded an area, but you started filling in your own spaces and losing points. You filled in so much that there was just one space left and your opponent could just play there and capture the group like this: https://online-go.com/learn-to-play-go#capturing-stones4

Try doing the tutorial again. Also, welcome to OGS!