Definition
One will find what one is looking for under an obvious sign for it.
Origins
From a traditional crosslike mark to indicate buried treasure.
How People Actually Use It
What is said upon finding your target has been marked out. Derived from an X on pirate treasure maps. The phrase was put into common usage by the British army, who performed executions by marking a piece of paper with a black x and positioning it on the heart of someone sentenced to death. The acting officer would say "X marks the spot" and the firing squad would shoot the x.
"X marks the spot, so let's go and dig up treasure."
Source: Urban Dictionary