Generally you adjust them to your zones, then you use the monitor to tell you your heart rate, what zone you are in, and possibly alarm you when you exceed a certain rate/zone. You usually do all that setting up of stuff ahead of time, so when you're riding most days, you just put on the heart monitor and use it to see how hard you're working (or overworking, or not working, as the case may be). That would probably be where you start anyway, though some HRMs have a test you can run through that it uses to predict your zones, or if you already have some sense of your zones and relatively how things work, you can use the HRM as feedback and then dial in the data as you go.

Features add $. If you want calories burned, customizations, GPS, add-ons, that all adds $. A basic heart monitor will work no matter what you're doing - you just strap it on and go. Polar has some good base models that do that (and many that do that+calories), they've been into the heart monitor thing for a long time, and you should be able to get one for <$100.