It means that you should, as and when you have time, write scripts with lines with and without a semicolon to see the difference it makes.
If you put a semicolon at the end of the statement, the output of that line will not be displayed in the scilab console window.
If you want the output in scilab console after running the file (program), then you have to enter the variable to know the output of that variable.
may be sometimes if you have single output, and the file has single line it will display.
For example, here the output will be displayed as 12. May be the version of scilab also matters.
a = 3;
c = 5;
d = a+b+c;