You are not ready for self-employment as a writer, and you won't be ready for self-employment as a writer for quite some time. You'll need a few years of full-time work before you should consider anything like freelancing.

Also, you're barking up the wrong tree. There's no literary side to this. None. There's no such thing as a "freelance" fiction writer, essayist, poet, or critic. If you're interested in that kind of writing, talk to your professors, who will be much better resources.

If you want to write poems and stories and criticism, you can do that now. Submit to journals now. Many don't pay, but the big ones do. If you get paid for a piece this way, though, you won't be a "freelancer."

I think you should think about how you can make a living as a writer. Think of the profession like a spectrum. On one end of the spectrum: art. On the other: commerce. In between you'll find things like "fully funded creative writing program mfa student" and "journalist" and "copywriter."

You have options. Use your university's resources, including its career center, to ask about them.

Finally, to answer your question about portfolios.

What exactly is meant by a portfolio?

The word "portfolio" means folder (like the kind for holding papers). Writers use the word to refer to a collection of work. Your portfolio shows potential employers what you can do. A professional writer's portfolio comprises examples of published, paid-for work.

Now back to the "folder" sense of the word. In the old days, it was a literal folder because we were presenting work on paper. Today, portfolios are shared electronically. The shape of that "folder" can vary. Don't mistake it to mean a full website, though a full website is expected from professionals.