Like a few others have said, it's likely just a temperature limiter. I don't know what make/model shower valve you have but they are pretty much all the same. You can see the instructions for setting this on a Moen in step 5 of this document https://www.google.com/url?q=https://www.moen.com/shared/docs/instruction-sheets/ins2153c.pdf&sa=U&ved=2ahUKEwjc-O2mgYb7AhXprIkEHWMUBpcQFnoECAoQAg&usg=AOvVaw1XFxpNUNJkcchf5HjvznbD. YMMV