The defender is thinking of a secret word. This word must be Scrabble-legal - see https://chat.stackexchange.com/transcript/53490?m=39476634#39476634. They reveal the first letter of it (and will later be forced to reveal more). Their goal is to last as long as they can by "defending" against people guessing the word.
Everyone else in the game is an "attacker". They give clues to words starting with the revealed parts of the word. For instance, if the defender has given up COM so far, clues may be for COMPUTER or COMA.
@Deusovi Couple things otherwise I think it makes sense to me
Shouldn't it be: Once the defender gives up on a clue, if any attackers have 'made contact' with the clue, they can say the answer. (Each attacker only gets one guess.) If any of the attackers' guesses are correct, then the attack "hits".
(from the quora post) The [attackers] can use any word they want, including phrases and proper nouns. They are welcome to give obscure clues that the [defender] [...] is expected to know, but doesn't for some reason. No advanced math if you're playing with 8th graders! And no asking me about Pokemon if you've already used two Pokemon clues and have established that I know nothing about the subject.
I think those points are really worth mentioning. The attackers shouldn't give clues that are merely objectively difficult, as then nobody contacts anything and the game grows stale quickly. They should give clues that they hope other attackers will guess, but the defender won't.