All of the information you can give is helpful when asking a question.
There is tons of knowledge on this forum but they need to have details to be sure they're answering the right question.
Is that an SR40C? If it is......
It's called the guide rod assembly. Best I can remember it's made of plastic.
I had an SR9C for a short time. A friend still has one. Both of them had some looseness/play in the guide rod assembly at the end of the slide.
I believe this is a normal condition for the SR40C and the SR9C.
According to shopruger.com both pistols use the same guide rod and they sell em for 5 bucks.
If it's not an SR40C....... disregard my comments.
Like MagnumBlackhawk said,if that's a SR40c or SR9c then it's normal to have a little movement in it. If it's not 1 of those then I'm not positive but pretty sure it's normal because like was said before,it looks like the end of the guide rod assembly. (Don't quote me on that though since I'm not positive what gun it is )