咕咚 is an onomatopoeia for the sound of a heavy object falling to the ground or splashing into water (like 'thud' or 'plop' in English). It is also used for the sound of drinking quickly in large gulps. The word almost always stands alone, often followed by 一声 or used as a sentence-initial exclamation. You will encounter it most in narrative and literary descriptions, not in everyday conversation.