Ok kiddo, let's first start with understanding what a lie algebra is. A lie algebra is a math concept that helps us understand how to apply algebra to study certain structures like groups, matrices and vectors.
Now, let's talk about what it means for a lie algebra to be orthogonal. This means that the algebra has a property of being "perpendicular" or "at right angles" to each other. Think of a square: each corner is at right angles to the other corners, and this is similar to how an orthogonal lie algebra works.
Lastly, let's discuss what it means for a lie algebra to be symmetric. When we talk about symmetry, we mean that an object has some kind of "mirror image." For example, the letter "B" looks the same when it is flipped horizontally like in a mirror. Same with a butterfly. Symmetry is important in math because it helps us understand relationships between objects that look alike.
So, when we put all of these things together, an orthogonal symmetric lie algebra is a math concept which describes a structure that is "perpendicular" to each other, has some kind of "mirror" image symmetry, and is used for understanding certain groups, matrices and vectors.