X = [X1 X2]; - Please Explain

I am beginner of Matlab, Please explain the following line. X = [X1 X2];

The forum is not the right place to explain the basics. The Getting Started chapters of the documentation explain them exhaustively and therefore it is strongly recommended to read them carefully.

Answer 1

X1=12
X2=24
X = [X1 X2]  % create a line vector X by horizontal concatenation;
Y=[X1;X2]    %vertical concatenation

You have to read this

http://www.mathworks.com/matlabcentral/answers/8026-best-way-s-to-master-matlab

Answer 2

That is the same as horzcat; see http://www.mathworks.com/help/matlab/ref/horzcat.html