An upper circuit is the top of a stock's daily price band. When the price rises to it, the exchange stops accepting orders above that level. Buyers queue at the circuit price but no one is selling, so the stock is frozen for the rest of the day. It usually follows big news: a large order win, a takeover offer, a favourable ruling, or in small caps, coordinated buying by operators. A stock hitting upper circuit repeatedly looks exciting and is where many beginners get trapped: they place buy orders each morning, finally get filled on the day the operators sell, and then watch it hit lower circuits on the way down. If you hold a stock that hits upper circuit, you can sell into the queue of buyers. If you want to buy, you often cannot, and that is a hint to be careful.