In most cases, .* does the same thing as *. The difference only applies to multiplying two matrices. Whereas * uses the linear-algebra definition (see its article for details), .* does the simple thing and multiplies the matrices element by element (obviously, they must match in size for this to work).
:[1,2;3,4] .* [a,b;c,d] [a 2*b] [3*c 4*d]
240 - Dimension mismatch happens when the matrices being multiplied don't match in size.