Legendary weapons are purple-grade guns, the highest tier before Exotics. Every player in Destiny 2 carries at least 2 of them all the time, so should you!
However, unlocking process for these guns is problematic. Firstly, most of the good legendary weapons has a low chance of random drop during specific activities. Secondly, godrolls for these weapons are even more rare. Buy Legendary Weapons boosting and you will be able to overcome those problems. save your free time and get a desired gun as quickly as possible.