What is meant by 100AT/125AF in a circuit breaker?

100AT =100 Amperes Trip

125AF =125 Amperes Frame

This means that the circuit breaker is rated for a maximum current of125 Amperes and is currently set to trip (Open Contacts) at100 Amperes.

AT - donates for tripping rate for breaker

AF- donates r frame size , physical dimensions to allow for replacing breakers in place of each others , manufactures , usually make 5 to 6 frame sizes ,

Example : 100 AT / 250 AF , 125 AT/250 AF , 225 AT/250 AF , 250 AT/ 250 AF

the rate of breaker is 160 AMP.

but the frame size is same as 250 AMP. meaning that you can simply replace the 16 A breaker by 250A breaker

From Clarity point of view, I would to recommend Jalal's explanation, however it is a minor  typo perhaps might need to be corrected in the last row of his comment 160A instead of 16A.