Overpass API is an interface provided by OpenStreetMap to filter data. We use it to query OpenStreetMap for relevant data to display POIs (Points of Interests) on the maps on demo.transformap.co.
To build a query for all community gardens, you use this Overpass Query:
Learning to use this language enables you to build custom maps out of data stored in OpenStreetMap.
A very detailed guide to the Overpass Query Language can be found in the OSM-Wiki.