A line dance is a type of choreographed dance where people dance in a line formation, either by forming a line or in two or more lines in front of each other or side to side. There are many dances that fit into this category: the conga, for example, is a line dance but so is Haka, country Line dancing, square dancing or some Polynesian traditional dances.
This type of dancing is not usually practiced out of very particular social contexts: you will very rarely see a conga line or a square dance formation at the club. Classes tend to be rare, but you might be able to find some if you live nearby a professional dance school, or you can learn choreographies in sites that offer on-demand dance lessons such as Steezy or Dancers.
If you want to practice what you have learned in class, country dance bars are probably the most popular place for this type of choreographed dance!