Hello Hussein.
The match not criterion for a class map matching statement essentially says “anything that doesn’t match what follows”. It is similar to “not equal to” in programming or logic. If we use the example in the lesson, and the command entered was:
R2(config-cmap)#match not access-group name TELNET
then the result would be that the policy map would match everything EXCEPT what is found in the access-list named TELNET.
In other words, the policy would match everything and would not match anything using port 23.
It is just another tool to be able to express what you require to be matched in the policy map and can be useful to more specifically define your requirements.
I hope this has been helpful!
Laz