A Viterbi decoder uses the Viterbi algorithm for decoding a bitstream that was generated by a convolutional encoder, finding the most-likely sequence of hidden states from a sequence of observed events, in the context of hidden Markov models. In other words, in a communication system, for example, the transceiver encodes the desired bits to be transferred, encrypting and at the same time preparing the encoded bitstream for an unfortunate data change caused by the channel noise. The receiver decoder knows the state machine created by the encoder hardware, which can find the most-likely transmitted bits based on the most-likely path.
A convolutional encoder is built the same as presented in the video: https://www.youtube.com/watch?v=oI0MwwXJ8dE&t=640s
The input bits shifts at different times (clocks) on the three registers shown in the figure above. Depending on the bits available inside those registers, the system will deliver a pair of bits produced (in this case) by two xor gates connected like so. The constraint length (k) is the number of registers an input bit can influence on the encoded bits, in the presented case k=3. The code rate (R) is a relation upon the number of bits that enter the system and the number of bits leaving, leading to R=1/2.