Responsive Web Design: What and how?

I'm just getting deeper into web design and I keep reading the term 'responsive' everywhere. I know this is a little bit of a broad question (please flag/comment/edit if it is too broad to validate answering here on stackoverflow), but what does 'responsive web design' actually mean? By which I mean, what are all the parameters and characteristics a web design needs to fulfill in order to be responsive? And how do I go about making an existing design responsive / whats the best way to set up your workflow in order to make your new designs responsive right from the start?