Two senses: (1) literal - can't physically get down (off a tree, stage, etc.); (2) idiomatic, with 脸 (face): 脸上下不来 / 下不来台 means 'can't save face, embarrassed in front of others.' The figurative use is the more common and more useful one to memorize.
grammar
Standard potential complement form: 下 (down) + 不 (negation) + 来 (come) - 'unable to come down.' Affirmative is 下得来 ('can come down'), and the simple action is 下来 ('to come down').
