Geo.PostGIS.st_distance_sphere

You're seeing just the macro st_distance_sphere, go back to Geo.PostGIS module for more information.
Link to this macro

st_distance_sphere(geometryA, geometryB)

View Source (macro)

Please note that ST_Distance_Sphere has been deprecated as of Postgis 2.2. Postgis 2.1 is no longer supported on PostgreSQL >= 9.5. This macro is still in place to support users of PostgreSQL <= 9.4.x.