Standard Time is a term coined by the Confederacy of Humanity, and later the Intergalactic Confederacy referring to various time frames all based around the rotation and orbit of the Sol System planet Earth. This term is used to simplify the time something is required for an object or individual(s) to accomplish an action.
For example: a standard year is made up of three hundred and sixty five standard days, made up of twenty four standard hours, which are made up of sixty standard minutes, which in turn is made up of sixty standard seconds. And so on.
Almost all human occupied areas use these time frames as a sort of "universal clock". making international, planetary, galactic, and so on, conversation easier to manage and understand. For example: If someone from a planet wanted to communicate when they would enter into a separate planet or moon, then they may say that they will be there in two standard hours from that point in time. Not all intelligent species use this system as they may have their own unique measurements of time.
This term is most commonly used to measure the amount of time a celestial body takes to rotate, or orbit its parent body.