Distros: "brands" of Linux; following UNIX tradition a no-frill (not user usable) version of Linux was floating in public domain, Value Added Resellers (VARs) like Red Hat, Debian and others pick that up and wrap a user-usable layer of software to it. Then you have a branded version of Linux those in the industry terms a Linux "distro". Going back to that "layer of software" it'll be somewhat different between distros as to what is needed as a user-usable layer for a particular purpose, but it is possible that someone would by-pass that and even turn a desktop distro like Fedora Core into a world-class web server machine.

Free: in the context of Linux and Open Source; this meant you have rights to the software complete with modifying the code and resell. Ask in return, you practice sort of academic honesty of "giving credit where it's due".

Kernel modules: drivers, most likely some piece of hardware but there is a small possibility that this module is for a software purpose.

Module probing: not really a term that you see on paper, just let you know Linux it has PnP capability about the same time Windows have. PnP means Plug-n-Play that let PC components work without prior hardware tweak.

Hotplug: strictly a generic term from USB, the verb is hotplugging, it is a name of the subsystem that Linux has for the use of hotplugging USB devices.