Multilinear polynomials are like big puzzles made up of smaller puzzles. Let's start with a simple puzzle: 2x. This puzzle has two pieces: the number 2 and the variable x.
But now, let's make a bigger puzzle: 2x + 3y. This puzzle has two smaller puzzles inside of it: 2x and 3y. These smaller puzzles only involve one variable each (either x or y), so we call them "linear."
A multilinear polynomial is a puzzle that has multiple linear puzzles inside of it. For example: 2x + 3y + 4z. This puzzle has three smaller puzzles inside of it: 2x, 3y, and 4z. Each of these smaller puzzles only involves one variable, so they are linear. But together, they make up a bigger puzzle called a multilinear polynomial.
Multilinear polynomials can get even more complex with even more puzzles inside of them, but they will always be made up of linear puzzles involving different variables.