Fabrik allows you to collect your work together in a number of ways.
A project is a collection of media (images, videos and text). A project may be a piece of work, a campaign you've done, a photo story, a case study, art piece or series of pieces that belong together, or a group of items that all comprises a project.
A portfolio is a way of collecting projects together. In Fabrik your projects can belong to more than one portfolio. Examples of portfolios would be genres of your work, for a filmmaker perhaps commercials, music videos, short films, documentaries. Portfolios can also be years (all the projects you created in a given year) or can themselves be campaigns or clients, if you've done a lot of work for a specific job or client over time.
Ultimately, portfolios are used to organise your site into sections that you think make help your audience understand your Fabrik site. There's no right or wrong way categorise your work, only the way that best makes sense to you.
These guides might help with portfolios, and how to use them: