This lecture covers the properties of diagonalizable matrices, focusing on the matrices ab and a + b. It explains that the matrix ab is not always diagonalizable, while a + b is. The instructor discusses the eigenspaces of matrix a and the conditions under which a matrix is diagonalizable. Additionally, the lecture explores the eigenvalues of matrix a for different values of b, highlighting the cases where a is diagonalizable. The importance of basis vectors in determining the diagonalizability of matrices is also emphasized.