This is about how we measure the position on the surface of the earth, or possibly another planet.
If we start by making an assumption that the earth is a perfect sphere, in this case we can use Spherical Polar Coordinates, as defined on this page. We can define a position on the earth using two angles:
 Latitude  an angle which is +90 degrees at the north pole, 0 degrees at the equator and 90 degrees at the south pole.
 Longitude  an angle about the line between the north and south poles, with 0 degrees at the Greenwich meridian.
We can assign an arbitrary x,y,z coordinate system in the local frame of the earth:
so from this diagram we can see that:
z = r sin(latitude)
and if we are on the Greenwich meridian then:
x = r cos(latitude)
but if we are not on the Greenwich meridian then this has to be modified depending on the latitude, so,
x = r cos(latitude) cos (longitude)
the y can be calculated from:
r^{2} = x^{2} + y^{2} + z^{2}
therefore y = r *sqrt(1  sin(latitude)  cos(latitude) cos (longitude))
where:
r = radius of the earth (approx. 6378135 metres)
Ellipsoid
The earth is not quite spherical, it is slightly flattened at the poles relative to the equator, this shape is known as an ellipsoid. This is a closer approximation, but not perfect, so many ellipsoids have been defined for different regions of the world.
Earth ellipsoids supported by x3d
x3d Code  Ellipsoid Name  a  Equatorial Radius (metres)  f  Flattening 
AA

Airy 1830  6377563.396  1/299.3249646 
AM

Modified Airy  6377340.189  1/299.3249646 
AN

Australian National  6378160  1/298.25 
BN

Bessel 1841 (Namibia)  6377483.865  1/299.1528128 
BR

Bessel 1841 (Ethiopia Indonesia...)  6377397.155  1/299.1528128 
CC

Clarke 1866  6378206.4  1/294.9786982 
CD

Clarke 1880  6378249.145  1/293.465 
EA

Everest (India 1830)  6377276.345  1/300.8017 
EB

Everest (Sabah & Sarawak)  6377298.556  1/300.8017 
EC

Everest (India 1956)  6377301.243  1/300.8017 
ED

Everest (W. Malaysia 1969)  6377295.664  1/300.8017 
EE

Everest (W. Malaysia & Singapore 1948)  6377304.063  1/300.8017 
EF

Everest (Pakistan)  6377309.613  1/300.8017 
FA

Modified Fischer 1960  6378155  1/298.3 
HE

Helmert 1906  6378200  1/298.3 
HO

Hough 1960  6378270  1/297 
ID

Indonesian 1974  6378160  1/298.247 
IN

International 1924  6378388  1/297 
KA

Krassovsky 1940  6378245  1/298.3 
RF

Geodetic Reference System 1980 (GRS 80)  6378137  1/298.257222101 
SA

South American 1969  6378160  1/298.25 
WD

WGS 72  6378135  1/298.26 
WE

WGS 84  6378137  1/298.257223563 
We can assign an arbitrary x,y,z coordinate system in the local frame of the
earth:
so from this diagram we can see that:
z = a * (1 f) *sin(latitude)
and if we are on the Greenwich meridian then:
x = a cos(latitude)
but if we are not on the Greenwich meridian then this has to be modified depending on the latitude, so,
x = a cos(latitude) cos (longitude)
the y can be calculated from:
r^{2} = x^{2} + y^{2} + z^{2}
therefore y = a *sqrt(1  (1 f) *sin(latitude)  cos(latitude) cos (longitude))
where:
 a = radius (on semimajor axis) of the earth
 f = flattening = (a  b) / a