Not sure what this is???

This is in the basement floor next to our bathroom. Is it something we can use to tie in a new bathroom? What is is exactly?

unscrew the fittign and look in. looks to me liek a cleanout or maybe backwater valve

There is a bigger spot in our floor where the main line comes in. Is it safe to assume that, that would be where the backwater valve and this (item in picture) is after but right before bathroom and washer so may be a cleanout. Don't know anything about plumbing but would like a little info before we ever have anyone come it to see about adding another bathroom. Thanks

WE cannot tell what the "bigger" thing is, but that is NOT a typical cleanout plug. The correct way would be to have put the backwater valve on the plumbing for the basement and connect the upstairs plumbing past it so it could operate even during flood stage when the backwater valve is closed. We cannot "look" at your pictures and tell you how the installing plumbers did their work.