ETA SA is a special company that makes really cool things called watches. Watches are like little clocks that you wear on your wrist. ETA SA makes the parts that go inside the watches to make them work.
It's like baking a cake. ETA SA is the special cake mix that makes the cake taste delicious. The other watch companies just add their own special touches to make the cake look pretty and unique.
ETA SA has been making watch parts for a really long time and they are very good at it. Lots of watch companies use their parts because they know they will work well and keep the watch ticking for a long time.
So, when you see a nice watch on someone's wrist, remember that ETA SA might have helped make it possible for that watch to work!