A "shelf" is really nothing more than a registration statement of shares that can be made available at the moment the company needs them. Typically through a 42b offering but also could be a direct offering to certain investors/institutional investors.

A shelf can sit there for years and not be touched at all. It's there though, as a deterrent from a hostile takeover as well as being able to make a deal very quickly. Either for new hires or to further the pipeline.

imo