Generally, standard input, referred to as "stdin", comes from the keyboard.

When you type stuff, you're typing it on stdin (a standard input terminal). A standard input device, which is usually the keyboard, but Linux also allows you to take standard input from a file.

For example:

cat < myfirstscript 

                

This would tell cat to take input from the file myfirstscript instead of from the keyboard (This is of course the same as: cat myfirstscript).