Okay, imagine you have a circle and you need to draw a line that starts from the center of the circle and goes all the way to the edge. This line is called a radius. Now, imagine you need to draw another line from the center of the circle to the edge, but this time at a slightly different angle. If you draw lots of these lines at different angles, you get what's called a Bessel function.
But what if the lines you draw don't reach all the way to the edge of the circle? That's called an incomplete Bessel function. You can think of it like drawing lots of lines from the center of the circle, but they only go part of the way to the edge.
Incomplete Bessel functions are important in math and science because they can help us understand waves, vibrations, and other things that move in circles. They can also help us find solutions to complicated equations. But for now, just remember that an incomplete Bessel function is like a bunch of lines that don't quite reach the edge of a circle.