When the two bodies are close, the gravitational field is stronger, and the passage of time is slowed. With pulsars, the time between pulses (or ticks) is lengthened. As the pulsar clock travels more slowly through the weakest part of the field it regains time. This is a relativistic time delay. It is the difference between what one would expect to see if the pulsar were moving at a constant distance and speed around its companion, and what is actually observed.