The DM is rolling a d8 to determine which of 4 directions the PC charges in, with a roll of 7 or 8 corresponding to the direction of the goblin
Ben's answer accurately summarizes the premise/lead-in to the roll:
The Goblin was hidden, and none of the PCs knew where it was. One of the players attempted to spot the Goblin, but was unsuccessful, so the [next] player decided to charge "in the general direction". This is going off the location where they last saw the Goblin.
The DM's roll is to determine the chance of whether or not the Player finds the Goblin. There is a random chance - the player has no control over this, and this is often resolved by the DM.
In essence, Chris Dupuis's character is trying to charge at the goblin without knowing exactly where it currently is. The DM, Greg Bilsland, adjudicates whether he goes in the right direction.
The DM explains the roll you're asking about as he makes it:
You're looking for a, we'll say a 7 or 8 'cause you're kinda going off in 1 of 4 directions down the road.
It's a little hard to make out given the video quality, but the DM seems to roll a d8. This fits the explanation he gives as he makes the roll; he picks which of those 4 directions the PC goes in (or more accurately, whether the direction the character picks is the right one - he doesn't specifically explain which specific directions the other numbers correspond to).
The DM rolls a d8, and decides that a roll of 7 or 8 corresponds to the right direction. (He could just as easily have rolled a d4, and decided that a 4 corresponded to the direction the goblin was in - but a d8 is rounder and ostensibly easier to properly roll than a d4.)
And judging from the reaction of Emi Tanji (the woman sitting next to Chris), and the subsequent outcome, the DM does indeed roll the necessary number.