Its CIDR notation. So /0 means the subnet mask has no on bits and would read as 0.0.0 0 if you had a /1 that turns 1 bit on in the subnet mask, so it would be 128.0.0.0.

If i had a /24 which is the subnet mask used for most small networks like your home router. There would be 255 minus 2 addresses available for clients (phones, pcs etc) so the subnet mask would have 24 on bits and read 255.255.255.0, which you may be familiar with.

(Assuming you dont know much, not to insult you, you might know plenty), but when writing any kind of instructions or guides, i was always told to assume the reader knows absolutely nothing and miss nothing out.