What’s the meaning?
The explanation of the problem is very easy.
What’s the origin?
It’s supposed to be the explanation that Sherlock Holmes gave to his assistant, Dr. Watson, when explaining deductions he had made. But this idiom doesn’t appear in the Conan Doyle books, only later in Sherlock Holmes’ movies. Holmes used to say “Elementary” to Dr. Watson though.
And what about other languages?
In French, we have exactly the same expression: “élémentaire mon cher Watson”.