What does rehashing mean?
Once in a while, I see this:
*** Notice -- OperNick is rehashing server config file *** Notice -- OperNick is remotely rehashing server config file
And other variations on the theme.
This is an indication that the Mibbit staff is doing some maintenance to the irc.mibbit.net network.
Other networks and clients may have slightly different messages.
Is it important?
To the staff, yes. To general users, probably not.
A change to settings such as staff IP ranges, different server adds and moves, routing changes, etc etc. It's usually pretty boring stuff to general users.
Will you tell me why a rehash just occurred?
Well, sure. It's really not a big deal, but is kind of perplexing about why you would care, and if the sanitized answer you will get adds any additional knowledge.
We are not about to tell you "oh, sure, I just added 192.168.3.17 to Axod's allowed IPs for his O-Line..."
Can I block it?
Sure, you had to turn it on, it is not default, you turn it off mode s (server notices)
User snomask +s means that you can see network status messages called server notices. These are messages that the system and/or staff (Opers) considers important.
- To disable this notification, issue the command /mode YourNick +s -s.
UnrealIRCD User snomasks 
UnrealIRCD User and Channel modes 
UnrealIRCD User and Oper commands