"Please be careful"
조심 basically means caution. As a verb, it's a 하다 verb, which 하다 means "to do." In this example, 하다 is conjugated in the formal imperative, 하십시오. The "please" is loosely translated from the formality. "To be" in this case is "to do." You can think of the translation as, "Please use caution."
It's from the verb 조심하다 which means "practice caution" or "be careful". Tye ending here is (으)십시오 which makes it "do it" but in the formal polite way.
"조심" means careful and "하십시오" means please, so I think it literally translates to "Careful, please."
조심 = careful, 시 = please, and the rest is the sentence ending politeness/formality stuff which also makes it a verb (be).
I know Duolingo is messy and unmoderated, but 조심하다 is "to be careful/wary/cautious" and you can use the app Dongsa to see all conjugations of verbs
They're including the "please" meaning with the translation, so it has to be 조심하십시오 or 조심하세요