One design approach to create an echo effect is to use feed-back delay. In this case, the output of the effect is fed back through a delay line to the input of the system.
The use of feedback makes it possible to create multiple repetitions of the input signal. This is an important difference compared to the feed-forward echo, which only creates a single repetition on the delay line.
With feedback, each time a repetition is created, it gets sent back through the delay to create another repetition. This process repeats over and over again. In theory, this can create an infinite number of echoes.
Feed-back echo is not the only way to produce multiple repetitions. Another method is to create a multi-tap echo.