Here’s a review of the timers:
HSRP timers consist of the hellotimer and the holdown timer. Let’s say we have Router A and Router B functioning in an HSRP group where Router A is the Active router and Router B is the standby router. Timers by default are set to the following: hello = 3 seconds, holdown = 10 seconds. Hellos are sent every 3 seconds. If Router A goes down and stops sending hello timers, Router B will wait 10 seconds (the holdown timer) before becoming Active.
So the purpose of the hello and holdown timers is to essentially define under what conditions a Standby router becomes an Active router.
In order to understand the preemption delay, it is important to understand preemption. Using our example above, Router A is Active and Router B is Standby. If Router A goes down, Router B will become Active (after the holdown timer expires). Let’s say Router A comes back up. Router B remains Active UNLESS preemption has been configured on Router A, and Router A has a higher priority than Router B. If this is the case, Router A will be forced to assume the Active state and Router B goes into passive.
Now the preemption delay is a certain amount of time that must elapse before Router A assumes the Active state once again. So, contrary to the other timers, it is the amount of time a HSRP router with a higher priority waits before assuming the Active state after it comes back up.
Now you may ask, why is that important? When is it used? Well, when a router first comes up, it does not have a complete routing table. You can set a preemption delay that allows preemption to be delayed for a configurable time period, say 60 seconds. (The default is 0 seconds). This delay period allows the router to populate its routing table before becoming the Active router.
I hope this has been helpful!