Further explanation please

I  am hoping someone could explain to me in the following SNMP community string set up, what the
0.0.0.15 after the IP address means.

I was explained this senero:

Further, it's also best practice to restrict the source addresses from where you can make these sorts of transactions.

for instance, on a cisco router you might want something like this:

snmp-server community ThePasswordIsSecret RO 10

access-list 10 permit 192.168.33.128 0.0.0.15

This way only machines addressed within this range that know the community string can grab information about your gear.

As I am learning about community strings, I seem to be getting a little deeper in  other things.
This is a good thing for me. That is why I joined this site. It is well worth the money. And for those of you helping, I really do say prayers to the Lord for this site. But back to my question.

I am familiar with TCP/IP but not from the routing aspect where I am looking at something like this 192.168.33.128 0.0.0.15

Can some one provide an explanation and or a link to a site that can bring me up to speed pretty quick with a basic understanding, as I know that there will be a lot more to learn on this.

Thanks in advance… Michael