下去 (xià qù) has two main uses: as a verb meaning 'to go down' (like walking downstairs), and as a directional complement after another verb meaning 'to continue an action' (e.g., 说下去 'continue speaking'). As a verb, it's more general than 下降 (xià jiàng, 'to decline, drop' — used for numbers or levels) or 降落 (jiàng luò, 'to land' — for aircraft). The 'continue' sense is different from 延续 (yán xù, 'to last, go on' — used for time or events, not actions).