Foswiki contribs extend the functionality of Foswiki, in a lower-level way than plugins and add-ons. They may also provide alternative implementations for sections of the Foswiki core e.g. user management, or when an extension just can't be implemented as a plugin because it requires very close access to Foswiki internals.
An example is the MailerContrib, which implements mail notification of topics changes.
Relevant links on Foswiki.org:
Foswiki:Extensions.ContribPackage - list of all contributed contrib packages
Foswiki:Extensions.ContribDevelopment - discussion and feedback on contributed packages
Foswiki:Extensions.ContribBrainstorming - open forum for new contrib ideas
Foswiki:Extensions.ContribPackageHowTo - instructions and template to create a new contrib package