America is referred to as a social experiment what do you think that phrase means

I understand it like this: The US was special in the way that it was a society created "from the scratch", and not by centuries of social negotiation. Also, it was formed by large group of immigrants that had to negotiate their live together and their structures in a very sudden way rather than evolving them from previous structures, as was done in Europe. This was both a challenge and an opportunity: it allowed to not be restricted by the social structures already there (such as nobility), but there was a risk of a "collapse" in that conflicts would arise and the society would not function.