HOW VRRP works
VRRP means Virtual router redudancy protocol. I am telling you in very general language not in too technical.
Assume that you have a two firewall in HA mode .
1 - Firewall ip is - 192.168.6.1 (primary)
2 - Firewall ip is - 192.168.6.2 (Secondary)
Now the firewall which is in standby that will look contineously ARP from the primary one and if the arp will not getting by the secondary firewall in given threashold time secondary firewall will become up and automatically primary will go to in standby mode.
The same thing will go on , now when the again arp will getting by primary firewall then automatically secondary will come as primary and primary will come as secondary.