those are fittings for draining and blowing out the system. there is supposed to be either 1/2" or 3/4" threaded plugs in each one.

But why the water came out from it in full force when I turned on my sprinkler system and it stops when I turn off my sprinkler system. My neighbor said that probably the part is bad. Look like there is a ball inside it Attachments are three pictures of it. Where can I buy this part?

that fitting is called an auto-drain and if you take it to any plumbing supply store they can get you the proper replacements.

Thank you for all replies. One more question. The water should not come out from it in full force all the time, is this correct? I used my sprinkler system every year; but I never see the water coming out like that all the time until now. The part look so simple.

I changed that auto-drain part and it works. As a matter of fact there is no need to put that part because I saw a lot of youtube video that did not have that part in their systems and with this part you are at risk of very high water bill when this part breaks down.

Oh I forgot to tell you that I broke one of the small pipes when I tried to remove this part. I was lucky that my neighbor (mechanic) helped me out for free to fix it for less than $10 by using 3/4" CPVC Thread Slip x Slip Coupling Fitting and JB Well. If these auto-drain parts break again, I will block those two pipe holes permanently. I don't mind to pay the cost of these two auto-drain parts; but I don't want to pay high water bill when they break down. Another thing after the fix, I noticed that the water came out from each sprinkler head stronger than before. Again thank you for your help.