"The long and short of it", also phrased as "The long and the short of it", is an expression commonly attributed to English playwright William Shakespeare. However, the Oxford English Dictionary (OED), widely regarded as the definitive authority on the origin of words in the English language, provides a citation of this phrase from an unidentified source that predates Shakespeare by a century: "Thys ys the schorte and longe."
The Bard of Avon used this phrase at least once in three of his plays: The Merchant of Venice, A Midsummer Night's Dream, and The Merry Wives of Windsor where he uses it twice. In all cases the expression is turned around from our common usage today, as in "The short and (the) long of it". The OED provides a source for the first use of this expression in the order of "long and short" as a gentleman by the name of William Walker who wrote it this way in 1690. It should be noted that both variations of this expression are still used and are considered interchangeable.
Modern dictionaries define this idiom as indicating that one is providing a brief summary of the most relevant information rather than going on at length or in detail. This meaning assumes that there is potentially a substantial amount of relevant or circumstantial specifics that could be conveyed, but that the conclusion or outcome is the same without needing to take any of that into consideration. This is somewhat narrower in scope than the OED's definition, that being "the sum total, substance, upshot". The OED offers up the additional use case where there are no other details to convey, the summary being all there is to say about the subject. This would be used in situations where the receiver of information might be expecting a more complicated or nuanced explanation, and the conveyor of information wants to make it clear that there is nothing "longer" than the "short" of what is being provided.
Sources:
https://english.stackexchange.com/questions/139789/the-origin-of-the-long-and-the-short-of-it
https://www.merriam-webster.com/dictionary/the%20long%20and%20%28the%29%20short%20of%20it