Have you ever deleted a user from Domino and added them to the Deny Access group only to have HR come back and say, “We've rehired John Doe and he needs back into Notes right now.” Of course you have, and if you've ever done this, you may have come across an issue trying to get the user connected back in after removing their name from the Deny Access group and trying to get the user setup in Notes again. This is because the NameLookup cache hasn't expired.
To get the user on to the server, you can issue the command
show nlcache reset on each server they need access to. So if you've got a mail server, directory server, catalog server, Sametime server, app server that they need to get on to, you'll need to go to each server's console and issue the command. Or, you can use the Send Command to Selected Servers dialog in the admin client to send the command once to all the servers you want.
Another use for this command is if a user changes their Internet password and it “isn't working” - submit the command and they'll be able to get in.