A file "entry" can be a newspaper clipping, an observation (field note), a diagram, a turn of phrase you might want to use, a passage from a reading, something one of your teachers said. Anything you want to put into you storehouse of ideas and source material.
Be sure to fully document things you put in your file. Date, source, why you put it in, etc.
Periodically, you should read through and rearrange your file as your interests and projects evolve.