高帽 literally means 'tall paper hat' — a reference to an old practice of forcing a humiliated person to wear a tall paper hat as public punishment. But in modern usage, the word almost always appears in the phrase 戴高帽 ('to wear a tall hat'), which means 'to flatter someone' or 'to butter someone up'. The image is that flattery is like putting a tall hat on someone — it makes them feel good but is often insincere. You'll hear it in social and workplace contexts when someone is being excessively praised.