> could someone tell me what do I get from this operation
Assuming your variables are scalar (1 value) or, when multipled, produce a scalar, scan_trous_carres0 will be an mx9 matrix of 0s where m is the result of the multiplication. Try it, as dpb suggested, or read about it in the documentation.
> I am trying to figure out why I am I multiplying the values inside the parenthesis
We certainly don't know why you're doing that 🙂. Apparently those 4 variables describe lengths or sizes of something and your output matrix needs to have that many rows. Are those values scalar? Vectors that, when combined, produce a scalar? We don't know.
This function is commonly used to preallocate loop-variables.
> and then I have the 9 outisde that parenthesis
Well, apparently you need 9 columns of 0s. size(x,y) produces a matrix of 0s of size x*y.
> What kind of matrix will I get?
A matrix of 0s of class double