When a government gets its troops prepared for battle, that's mobilization. It's also mobilization when you organize your family to throw a surprise party for your sister.
Preparing a group of people so they're ready to act at a moment's notice is what mobilization is all about. It comes from the verb mobilize, which literally means "to make mobile." Both words have been used in a military context since the 1850s, originally to talk about the Imperial Russian Army and its mobilization at the start of the Crimean War. Before that, mobilization simply meant "rendering capable of movement."