"Let's hide in the park quickly!"
Translation:공원에 빨리 숨어요!
It's kinda the same in English - Let's hide in the park vs. Let's hide in the park together vs. Let us hide in the park. The message is the same but the focus of the sentence is different. While Let's is just short for 'Let us' you can see Korean sentences like this one, whithout 우리, as equivalent to it. The 'us' is implied. (My native language is neither English nor Korean so take this comment with a grain of salt)