.shx

.shx

.shx is a part of Shapefile. Also see Shapefile.

Shapefile shape index format (.shx) The index contains positional index of the feature geometry and the same 100-byte header as the .shp file, followed by any number of 8-byte fixed-length records which consist of the following two fields

https://en.wikipedia.org/wiki/Shapefile#Shapefile_shape_index_format_(.shx)