basic shader questions

Hi

as a shader newbie, I want to use a existing shader but don't understand to begin those:

// textures

uniform sampler2D u_textureFg;

uniform sampler2D u_textureBg;

…

// alpha-blends two colors

vec4 blend(vec4 bg,vec4 fg){

…

How should I give a loaded texture to for ex:

uniform sampler2D u_textureBg;

Why is it later only named bg and not u_textureBg?

Many thanks