New to Forum/question on skimmer&drain

Good Afternoon everyone. I have a question regarding my pool. This is my first inground,,,came with the house we bought last fall.

I have a rectangular pool 16x32 with a deepend. I have a drain in the deep end and a skimmer at the opposite (shallow) end of the pool. I have a Heyward sand filter and super pump. 

The 2nd hole in the skimmer has a plug in it. the plug has a square top for a wrench and and what looks like a small nipple on the side. 

I dont know wether that plug is supposed to stay or be removed for operation. how can i tell if it is connected? and whether or not to remove it?

 

thank you in advance for the help

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