An internship is, most commonly, a 3-to-6 month period of time where you're working for free with the aim of learning.
Your work for that company will either be (a) generalized, working cross-laterally within the company or (b) specified, working on one project until completion or the end of your contract period, whichever comes first.
Internships are incredibly helpful and should not be limited to college. They allow for failures, learnings, and so much more, without the fear of termination unless you go against your signed teams of engagement with the company.
It is common to complete 1-4 of these throughout your undergraduate and graduate degree pursuit.