Hard Seltzer is a flavoured, alcoholic carbonated water beverage that is typically brewed from a sugar source (such as cane sugar). It is also sometimes made from a distilled neutral spirit.
Hard Seltzer is relatively low in calories and so it is often perceived as a healthier (at least in calorie count) and refreshing summer drink.
Typically, the alcohol volume is between 4-5% ABV, but it can often be lower as well as higher.