This error is happening when you have made local changes to files that haven't been committed yet. In git's parlance, you have uncommitted changes in your working tree.
When you are in this situation and you try to pull, git is not sure what to do with the local changes you have. Should it discard those and pull the changes from the remote? Should it commit those before pulling the changes from the remote? That's why it fails.
To avoid this problem before you pull changes into your local repository you either have to commit your local changes, stash them, or discard them. Once you don't have pending local changes in your working tree, you should be able to pull with no errors.