The verb saturate means to cause something to be fully soaked to the point where it can't take on anything else. A heavy rainstorm can saturate the ground, leaving puddles on the lawn because no more water can be absorbed.
The word saturate comes from the Latin word saturatus, meaning "to fill full, sate, drench." Saturate is often used to describe the aftermath of a big rainstorm, but other things can be saturated as well. If you're a workaholic, you might saturate all your free time with work, leaving no time to spend with your family. In chemistry, saturate describes causing one substance, like a solution, to take on the greatest amount of another substance.