250 is the port. You can use statics for a one to one NAT or for Port address translation.
if you only had one public IP but 3 internal servers that needed to be publically accessable, you can use the port to determine which internal server gets the traffic.
PAT Example: for a web server, mta, and a POP3 server you could do the following:
all other outgoing traffic would use the global nat.
One to One NAT Example:
static (inside,outside) 24.113.x.x 192.168.1.5 netmask 255.255.255.255 0 0
The last 2 items (0 0 in my example) are the 'max connections' and the 'embryonic limit'.
Max connections in the maximum number of connections you will allow at any given time.
Embryonic limit is the maximum number of connections you will allow at any given time that have not yet completed the handshake between the source and destination.