Questions about some parts in editor

Parallax: As the viewpoint of the player moves side to side, the objects in the distance appear to move slower than the objects close to the camera. Para X and Para Y define how fast the background layer moves (x-axis and y-axis).

clipping: means that only visible objects will be calculated. If the teeworlds sun in the background moves out of the visible range, the pc still calculates where it is supposed to be rendered. If you use clipping you define an area within the sun will be rendered.

shift usually means that you can change the value more exact as the values change more slowly.

You flag the layers to be details. If the low details option is checked in the graphics options these layers which are not set as details won't be rendered. This is an option to increase performance on slow pcs.
E.g. all standard maps will only show the main tiles and one background if you details are disabled.

the first one is the time, the second one is either the color or the position (depends on the kind of envelope)

yes

you can set a delay of the envelope,
e.g. +100 will delay the start of the envelope's animation by 100ms