As a marketing intern, you'll have a hand in helping a company improve its marketing and advertising strategies. You'll gain a lot of helpful experience and definitely learn a lot about marketing best practices during your internship.
Also, during your internship, you may learn about marketing needs and the use of designing specific materials for marketing efforts. You'll probably get to work alongside regular team members and get to evaluate the company's current marketing efforts.
While internships don't typically last longer than a couple months, you'll probably be able to treat this experience like a part-time job for the time being. If you do a nice job during the internship, you might even find yourself with a job offer by the end of it.