Domains are the letters that come after the name of a URL.
For example, www.easternflorida.edu
The ".edu" is the domain of this website.
Not all domains are the same in terms of containing quality sources for you to use in your papers / projects.
For academic research, to find the most credible and accurate sites, you can use:
.edu (schools and educational institutions)
.gov (U.S. government agencies)
.org (official organizations, mostly)