Short for standard input, stdin is an input stream where data is sent to and read by a program. It is a file descriptor in Unix-like operating systems, and programming languages, such as C, Perl, and Java. Below, is an example of how STDIN could be used in Perl.
print "Type something: "; my $example = <STDIN>; print "\nYou typed: $example\n";
In the example above, when ran, the script asks you to type something. Whatever is typed is saved in the $example variable and then printed as "You typed: xxxx," where xxxx is what is saved in the variable.
For more information about stdin, visit the pages below.