The Enigma is the best-known rotor cipher machine. Each keypress sends a current through the plugboard, then through three rotors in series, into the reflector, and back through the rotors again to light a lamp. Every keystroke also advances the rightmost rotor by one notch - and when a rotor steps, it can carry the next rotor with it. The result is a polyalphabetic substitution that changes with every letter typed.
Enigma signal path through rotors and the reflectorThe plugboard swaps pairs of letters before and after the rotor path, multiplying the key space. The reflector guarantees the cipher is symmetric: enciphering the same letter twice at the same rotor settings returns the plaintext, which is exactly what allows decryption with the same machine and daily key.
A rotor position is a 3-wheel setting, and with a chosen rotor order, ring settings and plugboard pairs the daily key space is enormous. That strength is also the weakness: the machine never enciphers a letter to itself, and its stepping pattern leaks enough structure for cryptanalysts - at Bletchley Park - to break it at scale.