Meaning
A face with downturned eyebrows, a small open mouth and a single drop of sweat on the forehead. Literally it is disappointment mixed with relief.
It fits a close call or a “phew, could have been worse” moment, like missing the bus but catching the next one. People often read the drop as a tear, so it is also used simply as sad. Example: “Missed the deadline by a minute, but the teacher still accepted it 😥”.