I am a bit confused with the Global Eurail Pass. If i get the 15 consecutive day pass does that mean for 15 days I can travel on Eurail trains as much as I want or like 1 trip per day? I don't quite understand how it works.
The basic answer is that it is truly a global pass in that you can hop on and off trains as much and as many times as you want over the course of that 15 days. Now read the fine print...some premium trains (the fast trains that there seem to be more and more of) may require a supplement and a mandatory seat reservation. You may also want to reserve a seat on the most popular trains at peak times at a cost. But overall, with some planning and willing to sacrifice a little time, you can get about anywhere. The details are pretty well explained in the "Railpasses" tab above or with the booklet that comes with the pass.
does that mean for 15 days I can travel on Eurail trains...
First, there are no such things as "Eurail trains". Eurail doesn't run any trains. Eurail is a company put together by the national railroads of Europe to package and market rail (Eurail) passes. You will be riding on one of the national railroads of Europe (French Rail, German Rail, etc).
And, yes, many, but not all, trains in Europe, require a substantial subsidy to ride on "premium" trains (Thalys, TGV, Ialian Eurostar, etc.).
And, yes, except for the supplements you will have to pay if you ride one of these premium trains, the Eurail pass basically gives you hop on-hop off access to all of the trains.
Just a note that in Italy, you can hop onto the Eurostar after paying the 18Euro supplemental fee. But, if you get off and then get back on a later Eurostar, cha-ching - another 18Euro. You can hop on and hop off the R trains in Italy. No extra fee.