Hi guys, I am a first year university student and busy with linear Algebra. Can anyone explain to me what on earth K is?

To give an example I saw some true and false exercises that say:

100

0k0 is an elementary matrix for all values of k.

001

and

k is any real number

I went through my textbook and google it but for some reason I just cant get an answer, can anyone please give some clarity on what k is?

Kind Regards

Hyperion