1. Texture co-ordinates. These always range from 0 to 1.
2. Always the top-left of the quad.
3. Yes: sometimes the effect will have been rendered to an intermediate window-sized texture if multiple effects are involved. Therefore the effect is processing the background on a sub-rectangle of the texture, not the entire texture. This leads on to 4: destStart and destEnd are vec2s marking the rectangle of the sub-texture being processed. Mix is indeed like lerp, and so mixing from destStart to destEnd by vTex gives you the correct texture co-ordinate of the background pixel to sample corresponding to the foreground.