< (Input redirection) - DOS/Command Prompt Reference
Passes content of the file, specified on the right side of this symbol, as an input to the command, specified on the left side.
<command> < <file-name>
By using input redirection, you can supply input data from a file to the command which usually receive input from standard input. Input redirection has a role to change standard input source to specified file (instead of console).
sort < document.txt
Sorts and outputs each line of text written in ‘document.txt’ by using Sort command.