Minecraft wiki entry for Coordinates, associated text: The x-axis indicates the player's distance east (positive) or west (negative) of the origin point—i.e., the longitude,
The z-axis indicates the player's distance south (positive) or north (negative) of the origin point—i.e., the latitude,
The y-axis indicates how high or low (from 0 to 255 (pre 1.18) or -64 to 320 (from 1.18), with 63 being sea level) the player is—i.e., the elevation,
The unit length of the three axes equals the side of one block. And, in terms of real-world measurement, one block equals 1 cubic meter.