Imagine you have some numbers - let's call them "complex numbers". Each complex number is made up of two parts - a "real" part and an "imaginary" part. The real part is just a regular number like you use in math all the time, but the imaginary part is a special kind of number that is often represented with the letter "i" (which stands for imaginary).
Now, a "multicomplex number" is just like a complex number, but instead of having just one real part and one imaginary part, it has multiple of each! So instead of just one number represented by "a + bi", we might have several numbers all added together, like "a + bi + c + di + e + fi", and so on.
This might seem confusing, but it's actually really useful in some areas of math and physics. For example, in quantum mechanics (which is a type of physics), the behavior of particles can sometimes be described using multicomplex numbers.
So while it might take a bit of work to wrap your head around what a multicomplex number is and how to work with them, they can be really powerful tools for some kinds of math and science problems.