The Universe is the totality of all space, time, matter, and energy. The Sun , Planets , Comets , Moons , Stars , and Galaxies are among the many items found in the Universe. Each day a different image or photograph of our fascinating universe is featured, along with a brief explanation written by a professional astronomer. I will expect you to know the sizes of a few objects. I expect you to be familiar with the Metric system, which is used throughout astronomy and in almost every country in the world.
Most numbers have a unit.
It tells the reader what the number is being used for length, time, weight, etc. The label inside the bracket is simply the shorthand way of writing the complete unit. Finally, you need to be familiar with the use of prefixes. The units of many numbers are abbreviated with a prefix to indicate how big it really is.