Transit timing variations refer to the changes in the observed time of a planet's transit across the face of its star, which can indicate the presence of other celestial bodies influencing the planet's orbit. These variations occur due to gravitational interactions with other planets in the same system, causing the transit times to differ from what is expected based on a simple, single-planet model. By studying these timing discrepancies, astronomers can infer the existence of additional planets and gather data about their masses and orbits.