market close jab hoti hy to price freez q hoti hy?

dear kia ap ny kabi note kia hy k jab market close hony wali hoti hy to prcie freez ho jati hy ap koi b order lagaty hain to wo order nai lagta chay wo sell buy order ho ya fr ap trade close karna chaty hun .. aesa specially stocks pe hota hy , mera sawal ye hy k aesa q hota hy? what is the reason behind that?