The term you are looking for is CIDR notation. The / is showing what the subnet mask is. So the subnet mask for a /8 is 255.0.0.0, which would be an extremely large network (over 16 million addresses, this was traditionally known as a Class A).
https://en.wikipedia.org/wiki/Classless_Inter-Domain_Routing#CIDR_notation
The term you are looking for is CIDR notation. The / is showing what the subnet mask is. So the subnet mask for a /8 is 255.0.0.0, which would be an extremely large network (over 16 million addresses, this was traditionally known as a Class A).
https://en.wikipedia.org/wiki/Classless_Inter-Domain_Routing#CIDR_notation