|If you don't understand what you've read using google I fail to see how we can explain it to you any better.|
The following link is what I consider to be a fairly clear, easy to understand explanation of a subnet mask and what it does.
Give it a read and see if that doesn't help to explain it better.
I'd try but I know my limitations all too well and I am not a teacher.
It matters not how straight the gate,
How charged with punishments the scroll,
I am the master of my fate;
I am the captain of my soul.