gl_MultiTexCoord question

Hi, I’m doing my first shader with GLSL, and I’m a bit conffused with this specific line in the vertex shader:

varying vec2 TexCoord;

TexCoord = gl_MultiTexCoord0.st;

Where comes from that “.st” ? I’m not using glMultiTextCoord in my code, so, my question is… How is the texture mapped in my object?