Matrix–vector multiplication

Okay, let's say you have a piece of paper with a bunch of numbers on it. This is called a matrix. Now, imagine you have an arrow with some numbers on it. This is called a vector. When you want to multiply the matrix and the vector together, you take the first row of the matrix and the first number of the vector and multiply them together. Then, you take the second row of the matrix and the second number of the vector and multiply them together. You keep doing this until you've gone through all the rows of the matrix and all the numbers in the vector.

Once you have all of these new numbers, you add them up and that gives you the answer. It's kind of like adding up a bunch of tiny little math problems to get one big answer. Matrix-vector multiplication is used a lot in things like computer programming, where you need to do a lot of calculations quickly and efficiently.