A= [aij] n x n lower triangular matrix

B= [bij] n x n lower triangular matrix

C= [cij] = AB

Proof cij= 0 for i < j or lower diagonal

cij= [ ai1b1j + ai2b2j + ...+ai(j-1)b(j-1)j ] + [ aijbjj +....+ainbnj ]

I am a little confuse of what they are trying to show within the 2 brackets.