If your currently open project is using Git as a revisioning system, the arrow in the gutter means that you deleted one or more lines at this location in the file.
If you commit (or roll back) your changes, the arrow is going to disappear.
Running a “git diff” on your file should show that you deleted some content between lines 95 and 96.
Depending on the theme you’re using in Atom, the arrow’s color might be different from the one in the screenshot.