2 beginner questions

[PavanneManor]

What is this thing? I can turn it on or off via a valve at the pump. Is it an aerator?
Also, I have some fine bubbles on one of my returns, does this indicate air in the lines? The pool was just refinished and has new plumbing.

 

[pjt]

What is this thing? I can turn it on or off via a valve at the pump. Is it an aerator?

Yes.

I have some fine bubbles on one of my returns, does this indicate air in the lines?

Do you have a saltwater chlorine generator (SWCG)? When it's generating chlorine, one of the byproducts is hydrogen gas which can be seen as small bubbles coming from the returns.

 

[Sandra B]

I had one of those in your first pic in my old pool. If you turn it so the opening points up, it shoots a fountain into the pool. Nice for cooling off.