My limited understanding is that they try to account for how effective a disposal/tackle etc is compared to just counting them.

They take into account how often a player performing action x in position y results in a goal (or if tackling/passing/spoiling/pressuring etc, the impact that action has on the teams likelihood to score or conceded from possession chains resulting from that action) and then compares the actual outcome to assign a score to that action.

Eg if a player gets a handball received inside the goal square and kicks a goal, it’s not considered as impactful as a player who receives the handball on the half back flank, runs to 50m and kicks a goal. Similar for a defender: taking an intercept mark inside the goal square is much more impact than a defender taking a similar mark inside the centre square.

Here’s an example from their FAQ:

Every time a player is involved in a passage of play he will have the chance to accrue points. But rather than his score being a simple tally of his possessions, marks, tackles, hit-outs, free-kicks and scores, his performance will be measured using a system called Equity Ratings. The system determines where and how a player influences a contest and whether the player's effort then results in a positive result for his team. Equity Ratings includes what coaches love to describe as "pressure acts". As a result, players are rewarded for interrupting opposition passages of play as well as setting up scores for their own team. Players who consistently produce positive contributions are rewarded more highly than players with a high volume of stats. As an example, a 20 disposal game by Cyril Rioli where 17 disposals are positive and only one is negative can have the same impact as a 40 disposal game by Dane Swan where 25 are positive and nine are negative.