What is monthly traffic, and how can I calculate this ?

You can calculate this by multiplying your average page size with the number of expected page views per month. If your average page size is 30KB and you expect 50,000 page views per month, you will need 0.03MB x 50,000 = 1.5GB.

Larger, commercial sites often consume more than 100GB of monthly traffic.

A small or medium web site will consume between 1GB and 5GB of data transfer per month.