Before understanding the coordinates you should know that the Minecraft map is divided into three parts. And those parts are referred to as X, Y, and Z.  And the coordinates sum up as X, Y, and Z. Where X, Y, and Z are some values that represent the location. Such as the X represents the East/West location on the map. If the value is positive it will expand the positions towards the east. And if it is negative it will expand the position towards the west. Y represents the location but of Upside or downwards. If the value is positive it will expand your position up and if it is negative it will expand its position downwards. And the Z shows the location of North/South. If the value is positive it will expand its position to the south. And if the value is negative it will expand the value to the north.  Let's take a coordinate as an example. The coordinates 200, -40, and 168 is a legit locations on the map. And it is a location of a waterlogged ancient city.