Every geometric shape has a spatial reference system associated with it. SRID 4326 represents spatial data using longitude and latitude coordinates on the Earth’s surface as defined in the WGS84 standard, which is also used for GPS.
Lat/lon still depends on a model of the earth. Typically, when people say lat/lon, the WGS84 reference geoid is assumed, but you can have lat/lon with a different geoid, hence the SRID 4326 that combines these two
To add o add an SRID to a shape you are defining with Well Known Text (WKT). In my PostGIS setup, I had to convert GeoJSON and WKT for a readble representation in requests.