Reference - Effects Panel

For moving trails it is quite common to assign a material that gradually fades to transparent over the length of the trail. The Effects panel contains controls that let you create and assign this sort of material in less steps then using the material editor.

The Apply Button

This button creates a new material from the settings you have selected and applies it to the GhostTrails object.

Apply Texture

If you check this box and select a bitmap file, the generated material will contain it as a Bitmap texture in the Diffuse map slot. The V Repeat setting will set how many times the bitmap repeats along the length of the trail.

Apply Fade

If you check this box, then the generated material will have a Gradient texture in the Opacity map slot arranged so that the trail fades to transparent along its length. The Midpoint value sets the Color 2 Position of the gradient texture.