For commands expecting the input to come from the keyboard, < can be used to get the input from a file instead of having to type it in. A simple example is
cat < fileThis will cause file to be displayed on the screen (the standard output). Note that it so happens that cat can take a filename as an input parameter, so
cat filewould result in the exact same response.
A more typical example is the use of a Fortran or C program (say prog) which takes data from the keyboard. If the user knows in advance what the data will be, the program could be run this way:
prog < datawhere data contains the data to be read by the program.