Notes

It's a scalar value that is a function of the entries of a square matrix.

It represents the area of the parallelogram defined by the vector columns of the matrix.

If the value is $0$, then:
 the column vectors are linearly dependent.

If the value is not $0$, then:
 the column vectors are linearly independent;
 the matrix is invertible.
Questions
 In one sentence, shows that $\det(M_1M_2) = \det(M_1)\det(M_2)$.