You will do reserve more regularly as a junior FO. Expect at least once a year. The more senior you are, the less frequently you will do it. This is basically a period of standby which lasts three weeks plus a set of seven fixed days off, although SHAG has different rules. If you think you're likely to be assigned reserve, you can bid in advance to get some control of your days off.
On shorthaul, some of it will be airport standby, some of it home standby. If I remember correctly, short haul has a 12 hour (?) period for airport standby or 1.5 hours from the car park for home standby.
For longhaul, all standby is from home with max 2 hours from the car park. In most cases, the company will try to give you more notice but if you live further away than that time, get yourself a girlfriend in Windsor.