Imagine you have a group of friends and you are playing a game where you all have to do different things together. However, sometimes one person might do something that messes up the game for everyone else. To avoid this, you decide to measure each person's actions to make sure everyone is doing what they are supposed to be doing.
In the same way, when mathematicians talk about a "measurable group action," they are talking about a group of transformations or actions that can be measured in a precise way to make sure everything is going smoothly. This group could be made up of rotations, translations, or other types of transformations that change the way a mathematical object looks or behaves.
By measuring the group action, mathematicians can ensure that every transformation in the group is doing what it is supposed to do and isn't causing any problems for the other transformations. This helps them to understand mathematical structures better and to find new ways of solving problems.
So, in short, a measurable group action is a way of measuring and controlling a group of mathematical transformations to make sure they are all working together properly.