Geo.WKB.encode_to_iodata

You're seeing just the function encode_to_iodata, go back to Geo.WKB module for more information.
Link to this function

encode_to_iodata(geom, endian \\ :xdr)

View Source

Specs

encode_to_iodata(Geo.geometry(), Geo.endian()) :: iodata()

Takes a Geometry and returns WKB as iodata (a sequence of bytes).

The endian decides what the byte order will be.