If you aren't a Destiny player then this might seem confusing to you. At its base level Destiny is a first person shooter, meaning you have guns and you shoot things with them (to put it the simplest way I can).
In Destiny there are 'raids' which typically involve 6 people taking on multiple enemies and bosses for 1-3 hours. The first people to do raids will typically take 6-12 hours to actually figure out how to complete it.
Crota's End is the latest raid for Destiny, it also usually includes 6 people to take on multiple bosses, Crota being the last enemy.
Many gamers take on these raids in interesting ways once they get used to the mechanics. I have seen runs with shotguns only, or hand cannons only. I've seen solo runs (meaning only one person will run through the raid, alone). This gamer decided to take on Crota's End raid without guns. This is an incredible feat seeing as the game is designed around shooting guns. He used a combination of his character's super power and grenades to go through the whole raid and take down Crota. This is awesome!