Slightly formal — common in writing, presentations, and editorials. Synonyms:
总之 (zǒngzhī, more conversational 'in any case/in short'),
总而言之 (more emphatic, 'all in all'),
一言以蔽之 (literary, 'in a word'). The
之 is the classical object pronoun 'it', so
简言之 literally means 'to put it briefly'.