AX is the accumulating register.
In this code, it is the sum of five numbers.
The five numbers are generated from the TextBoxes in dimava's serial program. (I'm not sure about the connection between the text in the TextBox and the number generated)
With dimava's algorithm, if ax = 19h, then the serial number is valid.
I don't know why, I didn't dig into this.
The point is, skip this jump and the program is cracked!

I use wdasm - it is more fun!