A reset can be considered to be the ultimate interrupt because it is non maskable interrupt mean it can not avoid when it is called, since no combination of bits in any register can stop, or mask the reset action, unlike other interrupts. RESET is actually external switch Whenever a high level is applied to the RST pin, the 8051 enters a reset condition and program will restart from address 0000h.
After the RST pin is brought low, the internal registers will have their initial value from where it start system.
In simple word, RESET does the work like rebooting system.
Above pic shows RESET circuit digragram.
In which we use a capacitor and basing register as shown.