My experience:

On the 2nd day that we were in Barcelona we got to see the busiest street in the city which is the La Rambla. We were on a gothic tour of the city when we arrived to La Rambla. I quickly realized that this street was designed to be a tourist attraction. It was crowded and busy with people who were shopping, getting caricatures, eating outside, or walking to their destinations. La Rambla is a street that has several little alleyway entrances so it's very easy to get on and off. As a group we spent a half hour or so looking at several different merchant tents then we were on our way to La Boqueria Market which is a popular destination off La Rambla. Overall, I was not too impressed with the street because I almost did not recognize it when we first arrived but it is a great place to shop!

A couple days later on our walking tour of Girona and Les Figueres I found out they also have a busy street that is referred as La Rambla. In the United States we have these busy streets usually referred as main streets or "downtown". I believe that the term "La Rambla" is just Spain's way of recognizing these streets.