The answer to your specific question is that due to the result of 8, which falls into the category 7–9, the villain gets to make an attack. That does not automatically mean that damage will be dealt. The GM has the choice to use one of the NPCs monster moves, which can be anything from simply doing damage or throwing a net at the PC to calling support. This entirely depends on the monster in question, and how the GM might have set up the monster with individual touch. Usually the monster moves are something that makes sense for that type of monster.
From the rulebook (page 56):
The enemy’s counterattack can be any GM move made directly with that
creature. A goblin might just attack you back, or they might jam a
poisoned needle into your veins.
In your specific situation, if the GM chooses to deal damage with the monster, the damage die of the monster is rolled and the PC is damage accordingly. However, damage is only something very abstract. If in the fiction the enemy does not simply deal some damage, but more like cleaves a bloody gash in your leg, this should additionally be taken into account in the fiction - like, it will be difficult for the PC to cling something or run fast until the wound is treatet.
However, I suspect that you want more than the answer to your specific question.
The general answer to the question "How does combat in Dungeon World work?" is that combat is a dynamic string of actions in the fiction that drive the overall scene back and forth. The players describe their actions, and according to the fiction moves are triggered. One of these moves is Hack & Slash. Depending on the results of the roll, everything might be fine (10+), there might be minor complications (7–9) or there might be major complications (6−). Depending on these, the GM has the chance to react to the players' actions with GM moves. These are specific moves unique to the GM that exist to drive the action forward. On a 7–9, as already established, the GM may use a monster move of the monster that was involved in the initial roll. On a 6−, the GM has the option to make a soft or hard move from the list of GM moves. These include the monster moves like above, but also environmental moves, such as dungeon and world moves, and a number of generic moves like use up their resources or separate them. Depending whether the GM chose to use a soft or a hard move, the consequences will be narrated in the fiction and then the players get a chance to react again, at which point the cycle starts again.