Okay kiddo, let's start with what a Lie algebra is. Imagine you have a group of people, like your friends at school. Now, if you can do something called "flip" where you switch two people in the group and it's still the same group, then your group has a Lie algebra!
Now we can think of the Lie algebra as a set of rules that describe these "flips" - we call them "Lie brackets". When we apply these rules to the group, we get a new group that's related to the old one in a special way.
Representation theory is a way to study these groups by looking at how they act on different things, like vector spaces or other groups. We call these things "representations" of the group.
Now, the representation theory of Lie algebras is about finding ways to represent the Lie algebra as a set of matrices that act on a vector space. This helps us understand the Lie algebra better and use it in different areas of math and physics.
For example, we can use representation theory to study the symmetries of physical systems, like atoms or molecules. We can also use it to understand the geometry of spaces, like curved surfaces or black holes.
So, in short, representation theory of Lie algebras is a way to understand how a group of "flips" relates to different things, and how we can use these relationships to learn more about the world around us.