A probability measure is a way of figuring out how likely something is to happen. Imagine you have a bag with 10 balls, 5 red and 5 blue. If you want to know the probability of picking a red ball, you can use a probability measure.
First, you count how many red balls there are - that's the "favorable outcomes". Then you count how many total balls there are - that's the "possible outcomes". So in this case, you have 5 favorable outcomes (the red balls) and 10 possible outcomes (all the balls in the bag).
The probability measure is just a way of comparing these numbers. You divide the number of favorable outcomes by the number of possible outcomes, and you get your probability. In this case, the probability of picking a red ball is 5/10 or 50%.
So a probability measure is just a tool we use to help us figure out how likely something is to happen. It's like a map that tells us how many different ways we could get to our destination. With a probability measure, we can make predictions and decisions based on how likely different outcomes are.