On that site, use the 'Find your code' button. Paste in the plus code, will display the location on map, can get the coordinates.
Or to do the decoding yourself, there is lots of implementations in different languages can use
As long as wanting to decode a 'full' 11 charactor or 'global' plus code, if wanting to decode shortened codes (a locatalty as a name, and 7 chars plus code) - much more complicated as need a geocoder. Not sure anyone has released a full decoder as opensource that fully supports decoding short codes.
Although there is an API if want that, means the complexity of decoding is offloaded to a remote server :)
powered in part by the Google Maps API.