Standard input (stdin) normally comes from your keyboard. Many programs ignore stdin.
E.g.
cat filename never reads its standard input; it reads the files directly. But without file names on the command line, UNIX commands that need input will usually read stdin.
Standard input normally comes from your keyboard, but the shell can redirect stdin from a file. This is handy for UNIX commands that can't open files directly.
E.g.
mail user@domain.com < filename. To mail a file use
< filename to tell the shell to attach the file, instead of your keyboard, to mail's standard input.
sed can read file directly, so no need to redirect stdin from a file. I hope this helps.