There are three reserve time slots that you could hold. 5am to 7pm, 10am to midnight, and 7pm to midnight. You are contactable during one of those times for the whole month (excluding your days off, obviously). Scheduling can change this on you, but it doesn't happen very often and it only affects you for one stretch of days at a time at most.

What happens after they contact you will vary based on what their needs are, but in general if you've got an early time slot (P1) you're not going to end trips late, and if you've got the second time slot (P2) you're going to finish later. So you get to decide whether you want to commute in the day before you've got reserve but probably get home the same day you finish, or whether you want to commute in the same day you start but probably not be able to go home until the day after you finish. That's not to say they'll never give you a trip that starts early and finishes late, or a trip that starts late and finishes early, but those don't happen as often.

The last reserve period is used for CDOs. Pros: you get to commute in the day of, and most of the time you get to commute home the day you finish. Cons: you don't build up flight time very quickly, and you're working the back side of the clock so it can really ruin your sleep cycle. Some like it, others don't.

Reserve sucks, but it doesn't last too long. The only thing you really have control over is when scheduling can start calling you on your first day. Beyond that you're at their mercy, so you just have to roll with it. FWIW, currently FOs are not flying much on reserve, so plan a lot of time idle in the crashpad.