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 to input and output pins depending upon the respective voltage levels.