Pre-delay is a gap of silence before the reverb starts. If a vocal has a 50ms pre-delay, the listener hears the pure voice first, then the reverb blooms. This keeps the vocal clear and upfront while still sounding huge.
The human brain is an incredible audio processor. To hear a sound as a separate echo, the delay between the original sound and the reflected sound must be at least 0.1 seconds (100 milliseconds). If the delay is shorter than that, the brain fuses the two sounds together, perceiving them as one elongated sound.