In the old days, we used a serial cable between 2 PIX firewalls, for failover. This special cable would be be configured with one end as primary and the other as secondary. Which ever firewall got the primary end of the cable, would assume that title of primary.
Then, a few years into it, they came out with lan based failover. Now the challenge was that if we use a crossover ethernet cable between 2 firewalls (now ASAs), one still nees the title of PRIMARY, and the other have the role of SECONDARY. That is what the command "failover lan unit primary" does, it tells the ASA what its title is. This line of the config doesn't replicate with the rest of the config. On a good day, the Primary will be Active, and the Secondary will be Standby. Even if you are doing ACTIVE/ACTIVE, one of the units will still have the title of PRIMARY and the other SECONDARY, regardless of who is active or standby for which contexts.
Using this for your prompt, will help out a TON for when you are at the console:
Keith-asa1(config)# prompt hostname context priority state