Slots come in "pairs". Each pair entitles you to perform a take off and a landing at specific times. The slot doesn't constrain the destination, that is down to route licences.
bmi have approx 59 pairs, Regional have none, they operate some of bmi's slots on a wet lease.
Slots are administered by a company called ACL. If you want to know more start with the link below:
Airport Coordination Limited - Heathrow Airport
That's about it really.