The part after the slash is how many subnet mask bits to use. Since the use of classless routing you use slash instead of saying class A or B whatever. Example:
192.168.1.1/24 is 192.168.1.1 255.255.255.0
255.255.255.0 is using 24 of the 32 bits to create the subnet.
in binary it looks like this:
11111111.11111111.11111111.00000000
so a /30 would look like:
255.255.255.252 or in binary
11111111.11111111.11111111.11111100
the remaing 00 is for hosts; the 1’s are the network.