Bandwidth is a measure of the amount of network resources you site is using. If your average web page is 25k, including images, and each day you have 500 visitors who each view 10 pages.
Your daily bandwidth usage would be: 25k x 100 visitors x 10 pages ea one views = 25mb
In one month, you would use 30 days x 25mb = 750mb
If your allowance is 1GB/mo, then you would be inside your limit.
To reduce bandwidth, you could make your pages smaller by reducing the size or number of images on each page.
Units of measure:
1 byte
1000 bytes = 1kb (kilobyte - actually 1024 bytes, but usually written as 1000)
1,000,000 bytes = 1mb (megabyte)
1,000,000,000 bytes = 1gb (gigabyte)
Although you can end up paying more for bandwidth, it usually means you have a very popular (and successful) site!