Basically, what it does is cache the config in memory, so that your "show run"s execute a LOT faster. Normally, everytime you do show run, it has to query EVERY interface in the system to determine the config for each, and then displays it.
This command tells the IOS to cache it the first time "show run" is executed, so every subsequent "show run" is loading a cache'd copy. Every time a config change is made, the cache is cleared, to keep itself fresh.
In other words, this command won't make a difference the first time you do a "show run" after a config change, but every "show run" thereafter(until the next interface change) will be loaded from the cache and be SUPER speedy.
For the impatient, the command is:
parser config cache interface