"해석해 주세요" is the correct way to say "Please explain it to me". 썸 is a recently coined slang meaning "an (early) romantic interest on (someone)". It comes from the first part of English "something", as in "There's something going on between the two". So 근데 요즘은 썸에서 끝난답니다 means "These days I only get as far as the early phase in the courtship". Or "Nowadays I can't seem to go beyond the early stage (of assessing each other". That is, the speaker just can't get any relationship going on to a serious stage". 썸 is actually used more often as 썸타다 - a verb. 타다 is "take a ride", "ride on something". It makes 썸 more interesting this way. So your sentences can also be said like 근데 요즘은 썸타다(가) 끝난답니다.