All your setting changes will appear in the Preview without affecting the image until you click on OK. Note that the preview displays the whole image, even if the final result will concern a selection. Don't keep Do Preview checked if your computer is too much slow.

Bumpmap: Relief is smooth and colors are preserved.

Emboss: It turns your image to grayscale and relief is more marked, looking like metal.

Azimuth: This is about lighting according to the points of the compass (0 - 360). If you suppose South is at the top of your image, then East (0°) is on the left. Increasing value goes counter-clockwise.

Elevation: That's height from horizon (0°), in principle up to zenith (90°), but here up to the opposite horizon (180°).

Depth Seems to be the distance of the light source. Light decreases when value increases.