Problem about textureProj

[apapaxionga]

textureProj performs a texture lookup with projection.

Can someone explain in detail for me about the function? i do not know what the "projection " mean in this sentence.

[ZbuffeR]

Read :
http://www.opengl.org/sdk/docs/manglsl/xhtml/textureProj.xml
The "projection" is the one happening with homogeneous coordinates (http://en.wikipedia.org/wiki/Homogeneous_coordinates).
This is needed to make perspective working correctly, allows to define texcoords at infinity, etc.