The game trims spaces and ignores capital letters before it compares. MOON and moon are the same word, and Moon with trailing spaces is the same word too.
Plurals are not the same word on their own. moon and moons are different answers, and so are rabbit and rabbits. The same-meaning confirm exists for exactly this case: if the room agrees a plural carries the same idea, every player taps the confirm button and the round counts.
Synonyms get the same treatment. ocean and sea are different words, so the game cannot score them as a match by itself; the room has to agree. That is why the confirm step asks every online player, not just the host.
A typo is treated as its own word. If somebody writes moom, the game compares moom; the room can still confirm that the typo means moon, and the round counts as same meaning.
Words can be 1 to 24 characters and may use letters, spaces, hyphens and apostrophes. If the word box rejects something, that is why. A player who submits nothing simply does not appear in the reveal; the round is scored on the words that were sent.