Configuring Layer 3 Redundancy with Hot Standby Routing Protocol – HSRP
HSRP states

A router in an HSRP standby group can be in one of the following states: initial, listen, speak, standby, or active.

When a router exists in one of these states, the router performs the necessary actions required for that state. Not all HSRP routers will transition through all states. For example, a router that is not the standby or active router will not enter the standby or active states.

HSRP Initial State
All routers begin in the initial state. This is the starting state and indicates that HSRP is not running. This state is entered via a configuration change or when an interface is initiated.

HSRP Listen State
In the listen state, the router knows the IP address of the virtual router, but is neither the active router nor the standby router. The router listens for hello messages from those routers for a duration called the hold time which can be configured. The purpose of this listening interval is to determine if there are active or standby routers for the group. Then this router will join the HSRP group, based on its configuration.

HSRP Speak State
In the speak state, the router sends periodic hello messages and is actively participating in the election of the active router or standby router or both. A router cannot enter the speak state unless the router has the IP address of the virtual router. The router will remain in the speak state unless it becomes an active or standby router.

Standby State
In the standby state, because the router is a candidate to become the next active router, it sends periodic hello messages. It will also listen for hello messages from the active router. There will be only one standby router in the HSRP group.

Active State
In the active state, the router is currently forwarding packets that are sent to the virtual MAC address of the group. It also replies to ARP requests made of the virtual routers IP address. The active router sends periodic hello messages. There must be one active router in each HSRP group.