Hi can anyone look at the attached picture and tell me what is this cap call(circle in red)? and what is this metal tube for?
Just realize today, I am missing this cap. Is it important to have it?

Thanks for your help

 

That is the low pressure side of the refrigerant line.
That cap is protecting the valve used to recharge your A/C.
Not a BIG deal, but you really should have that covered.
You also don't want dirt/debris to collect inside there and contaminate the refrigerant if you ever need to get it recharged.
Best to keep that inner area clean.

 

thanks for your help~