'No frills' does, as you suggest mean like easyJet, Ryanair or any other budget airline. No films, audio channels or meals (unless you pay for them).
I'll explain a charter flight by using a bus analogy. To get from A to B you can use a scheduled bus (or coach service). But someone might also hire a coach, just for one day (or at regular intervals), and sell tickets on that coach. If you used that service you'd be buying your ticket from the hirer, rather than from the firm that owns the buses.
Charter flights are like that. (i.e. you're buying your ticket from a company which charters the plane). They used to have lots of rather odd rules. (e.g. charter operators had to provide accommodation with their flights, even though nobody wanted to use it. So you'd be given vouchers for a cheap & filthy hostel, hundreds of miles from your arrival airport just so that the operator could legally sell you the tickets). These days the rules are less restrictive but you can still only buy return flights (no singles), originating in the UK. Since the flights tend to operate on only one day per week, you have to book for 7 or 14 days (you can't book for, say, 9 or 10 days).
Chris