What does the in-game score in Age of Empires II mean?

Solution 1:

The first number is you personal score at that moment. The second number is the average score of the team you are in at that moment. If you are not in a team then these numbers will be the same, which makes it quite pointless to show it twice in my opinion.

This only works for teams that where made before the game starts. If you and your new ally for instance both set each other to ally it will update, meaning it will not show the average score between you and your ally.

For information about how the scores are made up: Age of Empires 2 Score system?

Solution 2:

Points are given for creating a unit (based on how many resources it costs) and losing the unit will have those gained points deducted. Same goes for killing units and buildings (based on their resource cost).

You also get points for gathering resources and exploring the map. Also looking at the summary page can sum it up well.