The backquote (`), which is quite alien to most Windows users, is the key just below the top left hand side ESC key on your keyboard.  This is a useful key to use when you want to use the standard output of a UNIX/LINUX command as part of another command.

For example, if you want to output the results of an addition.

# expr 10 + 42
# 52

If you would like to make it fancier,

# echo “The sum is `expr 10 + 42`”
The sum is 52

Note the backquotes (in red).