I purchased a National Geographic 76/700 EQ telescope and there was a piece missing. I'm wondering if it's needed or if I should return it, if you could also tell me what it does that'd be great.
It looks likely to be one of the bolts for adjusting the polar alignment of the scope. There are usually two opposed hand operated bolts for fine adjustment of the left/right pointing of the mount on the tripod (so it points north/south - you're expected to position the tripod so its roughly correct to start with), and another two opposed bolts (might only be one in some designs) for tilting the top part of the mount up/down so that the rotation axis in the top part (parallel to the telescope tube in the photo) (which pivots around the silver bolt left and level with the top of the red circle) points at the celestial pole (how much tilt this needs varies with your latitude).
If that bit is missing - (it looks most likely to be the one you screw in to tilt the top part of the mount upwards more) - you won't be able to set the scope up properly.
If you check the instructions on how to set the mount up, you should be able to tell which bit it is.
(The reason why it's important, is that if you set the mount up properly, then when something starts to drift out of view due to the earth's rotation, then you can keep it in view by only adjusting one of the slow motion controls).