Semi-aniline leather is a type of leather that is dyed only with soluble dyes. It is similar to aniline leather, though it is only slightly pigmented. Semi-aniline leathers allow the natural surface of the leather to show through (blemishes, cuts, etc.).
It is then finished with a thin protective coating to help prevent fast wear of the leather, or any other staying or discoloration from showing up on the surface.
Generally only higher quality leathers are used since they have nice, even surfaces. This can be a visually appealing dye approach, since it does allow the original surface of the leather to show through.