what does this command means?

for i =1: size(Training_String,2)

Answer 1

[CS Researcher]

Assuming Training_String is of size M x N, the command runs a for loop where i goes from 1 to N. Basically a loop which runs N times.

Answer 2

[Image Analyst]

The size() function gives the length of the dimension you give as the second argument. 2 means columns. So the loop will do as many iterations as there are columns in the Training_String array.