Imagine you have a bunch of crayons of different sizes and colors, and you want to color a picture. But instead of coloring random spots, you want to color only certain areas that are marked on the picture.
In the same way, the Lebesgue-Stieltjes integral is a way to calculate the area under a curve, but only in certain specific areas.
Here's how it works:
First, imagine you have a line graph with a curved line. This line represents a function - let's say it shows how much money you have in your piggy bank over a period of time.
Now, let's say you want to figure out how much money you saved over a specific time period. In order to do this, you need to find the area under the curve between two points on the x-axis.
Here's where the Lebesgue-Stieltjes integral comes in. Instead of just calculating the area under the curve directly like you might do in school with a simple rectangle, this integral breaks the area down into smaller, specific parts.
It does this by using a "measure function" - a function that tells you how much "importance" to give to different parts of the graph. For example, maybe you care more about the areas when the savings were increasing quickly, and less about the part where they were decreasing.
By using this measure function, the integral calculates the area under the curve only in the parts of the graph that matter to you.
So to put it simply, the Lebesgue-Stieltjes integral is a way to calculate a specific area under a curved line, by breaking it down into smaller, important parts based on a measure function.