Some web hosts offer hosting packages with "multiple domains".
This means, basically, that you pay a single fee but get the right to create more than one website (say: one site about your hobbies and another site about your company —or, in the case of "reseller packages", one site for your client A, and another one for client B).
Each of these websites would have its own domain, e.g., "myhobby.com" and "mycompany.com".
Note that this is not the same as having multiple subdomains, e.g., "hobby.mysite.com" and "company.mysite.com".
In that example, you would probably want to keep the sites completely separate (i.e., on different domains), or risk looking rather unprofessional!
Occasionally, web hosts use the expression "multiple domains" to indicate that you can use multiple domain names to reach the same site (think of the additional domain names as extra "pointers" to the site).
This can be useful in certain circumstances; for example: if your domain name is relatively hard to spell and you also own popular misspellings ("myinsurance.com" and "myinsurence.com"), or if you have registered your domain name both with and without hyphens ("my-domain.com" and "mydomain.com").
However, it's not a good idea to register multiple domain names with the intention of submitting them all to search engines, in order to get more traffic —the search engines will likely see right through this strategy, and they may perhaps even punish you for it...