Time is a relative concept that varies based on an object's velocity and frame of reference, with faster objects experiencing slower time. It is measured in relation to celestial bodies, such as Earth's rotation around the sun and its axis. General relativity predicts that time slows near massive bodies, affecting how time is perceived in different gravitational fields. This results in time dilation, where time moves slower in stronger gravitational fields, such as near large stars or galaxies. Ultimately, time serves as a fourth dimension in space, functioning as a fundamental reference for understanding the universe.