The rank of a matrix is the number of independent vectors in the matrix. A vector is independent if it cannot be expressed as a linear combination of other vectors in the matrix.
For example, the matrix [[1, 0], [0, 1]] has rank 2, because both of its vectors are independent. But the matrix [[1, 0], [2, 0]] has rank 1, because its second vector is a multiple of its first vector.
refs: https://textbooks.math.gatech.edu/ila/index.html http://immersivemath.com/ila/