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.

(Y): The Y coordinate represents a distance along the vertical plane up or down from the origin, just like in real-world altitude values. A distance up from the origin is represented by a positive Y value, (+Y). A distance down from the origin is represented by a negative Y value, (-Y).