翻天 is a verb-object compound (离合词) meaning literally 'to overturn the heavens', but it's almost always used figuratively to describe an earth-shaking upheaval, a convulsion in society, or the overthrow of a government. It often appears in the four-character idiom 翻天覆地 (tiān fān dì fù, 'earth-shaking'). As a separable verb, you can insert aspect markers or complements between 翻 and 天, e.g., 翻了天 ('overturned heaven') or 翻不了天 ('cannot overturn heaven').