skalibs is a package centralizing the free software / open source C development files used for building all software at skarnet.org: it contains essentially general-purpose libraries. You will need to install skalibs if you plan to build skarnet.org software. The point is that you won't have to download and compile big libraries everytime you need to build a package: do it only once.
skalibs can also be used as a sound basic start for C development. There are a lot of general-purpose libraries out there; but if your main goal is to produce small and secure C code, you will like skalibs.
skalibs originally contained public-domain code, but the law in some European countries - including France, where the author lives - does not make it easy for an author to abandon all his copyrights. So skalibs now uses the maximally permissive ISC license: the details are on this page.
The point is to make sure that skalibs, binaries statically or dynamically linked against skalibs, and source code using parts of skalibs, can be redistributed with the least possible restrictions.
Here are a few other libraries originating from the same place as skalibs, i.e. people start to reuse and package, or rewrite, code from Dan J. Bernstein, and then patch after patch, addition after addition, the code evolves into a project of its own: