What is a portfolio in game development? Does it include programming or modelling and animation too? Can somebody explain it to me?
Basically, you want something that can show to an employer what you've done in your industry.
If you're an artist, you'd want to showcase art you've done. Things like sketches, college projects, work from previous jobs (assuming they allow you to post them.)
If you're a programmer it's usually links to personal (game) projects you've worked on and details about how you contributed to any published titles you may have contributed to. With programming it's less realistic to show actual code but if you've worked on something yourself and/or you've worked on something OpenSource, it's certainly something you should show if you're able.
The point is, you want something an employer can look through and see the kind of work you do, your quality, and get a feel for how you are in your field.
Portfolios are not unique to game development. Commonly called portfolios, or more technically "career portfolio", see this definition from wikipedia:
Career portfolios are used to plan, organize and document education, work samples and skills. People use career portfolios to apply to jobs, apply to college or training programs, get a higher salary, show transferable skills, and to track personal development. They are more in-depth than a resume, which is used to summarize the above in one or two pages. Career portfolios serve as proof of one's skills, abilities, and potential in the future.
Basically, it's a collection of things you've done. This collection has the purpose of making you look good to potential employers. So you want to put everything in there that shows off your skills and talents.
In many cases this is a digital portfolio. This allows you to put in code and digital art assets. Some people make a game to represent their portfolio, others will just have a collection of things they've worked on.
It's not required to have one. In fields where it's hard to convey in words what your talents are, it's very nice to have something like this.