7805 is a voltage regulator integrated circuit. It is a member of 78xx series of fixed linear voltage regulator ICs.
The voltage source in a circuit may have fluctuations and would not give the fixed voltage output.
The voltage regulator IC maintains the output voltage at a constant value.
The xx in 78xx indicates the fixed output voltage it is designed to provide. 7805 provides 5V regulated power supply.
Capacitors of suitable values can be connected at input and output pins depending upon the respective voltage levels.