"Light" in American beer parlance means it's lower in calories and alcohol content. They came about initially because there were a lot of reactions to changing diets for Americans. Lots of stuff happened at once, like a very powerful dairy lobby, increase in high fructose corn syrup, big pushes to gain weight basically. A lot of Americans actually took notice and started developing modern diet culture, and part of that was cutting out beer. Companies responded by offering beers lower in calories. They stuck around because they're less bitter.