I think they mean when it's multiple choice and you have to select "let" and " 's " separately.
This is actually incorrect. Either two words "let" and "us" (which doesn't sound natural) or one word which is a contraction "let's". The " 's " on its own isn't actually a word and shouldn't be treated as one.
... I suppose Duolingo needs to take some English lessons? Hey Duo, count the number of contractions I've used in this reply as examples.
P.S. apologies for the English grammar lesson. Back to Korean