忐忑 is a vivid reduplicative word (联绵词) that describes a state of inner agitation — your heart feels like it's bouncing up and down (the characters 忐 and 忑 literally suggest 'heart up' and 'heart down'). It's commonly used for the nervous, uneasy feeling before an important event or after making a mistake. You'll often see it in the four-character phrase 忐忑不安 (tǎn tè bù ān), which intensifies the meaning.