You have two options when it comes to setting the word to be guessed. You can either manually type a word into the text field, or you can have the game randomly pick a word.
Manually setting the word is useful if you want to have a word that fits into your club/sport/organisation/branding.
To have the game select a word for you, select the length of the word from the drop down. Every time the drop down updates a new is selected. You can also use the reset button to pick another word of the length indicated by the length dropdown.