The X axis of a profile shows the distance along the section line, starting at 0 and increasing along the entire length of the section line.
The units displayed on the profile are the X and Y units that correspond to the grid it was created from. If you wish the Distance units to be in other units (e.g. if your map is in lat/lon units and you want the distance to be in meters or feet), change the coordinate system for the map to a system with the linear units desired and then recreate the profile.
The Y axis shows the Z value of the grid at that particular point on the section line. The units correspond to the Z units of the grid it was created from.