It means :
a=[1 2 3; 4 5 6] ;
b=[1 2; 3 4 ; 5 6] ;
a(1):b(1)
i.e. it is taking a(1,1) and b(1,1) and trying to make a linspace from a(1,1) to b(1,1). As the respective values are 1 each, so you are getting only 1. Change b(1,1) t0 5 and see.
a=[1 2 3; 4 5 6] ;
b=[5 2; 3 4 ; 5 6] ;
a:b
