According to Universe Today, astronauts use radio waves to communicate with Earth and between craft in space. While these waves travel at the speed of light, the long distances involved can induce delays in conversation between Earth and astronauts on long-distance missions.
During the early spaceflight era, craft in orbit simply communicated to ground relay stations directly using standard radio equipment. The relatively short range left gaps in coverage, such as those that occurred during the loss of control emergency during the Gemini 8 mission.
In 1983, NASA began launching the Tracking and Data Relay Satellite system in order to provide a network of complete communication coverage and avoid dead spots that could delay crucial communications between astronauts in orbit and controllers on the ground. These satellites can relay voice communication between astronauts and any tracking station in the world, with only a quarter-second delay.
Long-distance space missions may encounter longer communication delays. For instance, radio waves take approximately 1.3 seconds to travel from the Earth to the moon, resulting in the frequent pauses heard on Apollo moon landing recordings. Astronauts on a mission to Mars could encounter delays of up to 21 minutes, removing the ability to have any sort of real-time communications with those back on Earth.