Hi!
In the broad sense - to throttle is to restrict. In networking terminology - to rate limit? ARP throttling would mean to restrict the number of arp packets. It depends on the context in which you are trying to define it.
Example.
A router could throttle ARP request if it had to route a packet to a destination which did not exist in its ARP cache. Instead of trying to send out an ARP request for every packet that comes in, we might send 1 ARP request for every x seconds
HTH
Eugene
