What are the lines in my viewfinder for?

My Nikon D7100 viewfinder has a bunch of grid lines, i know they must serve a purpose but do not know what that is as of yet

what are they for?

Those gridlines are there to assist you in composing your images. Read the instructions manual and also do an Internet search on photography composition, and you'll have a better understanding. Those lines assist you in using the Rule of Thirds (if you don't know what this means, please look it up on the Internet).

They're composition guides. There just there to help you take better pictures. If you look up "The rule of thirds in photography" it will explain it better.

The gridlines are there to help you "straighten" as you compose your picture. They're also helpful for composition using the Rule of Thirds. Remember, via your menu, you can turn those grid lines on or off.

They are to ensure you keep the camera level with horizons. Or straight against buildings.