What is this thing? What does it do?

[Glennifer]

This is my moms pool set up and I am just curious what is this things purpose? What is it called? See attached picture. Thank you.

 

[mas985]

Looks like a check valve. It allows water to flow in only one direction which for that one is downwards. Is that the spa spillover? They usually do that so a raised spa will not drain when the pump is off.

 

[rdbrad]

Check valve. Yes. I wish I had one on my setup. When the pump is off the spa water level drains down to pool level and causes the plaster in the spa to discolor by being exposed to sunlight.