The FHIR Mapping Language is a part of the FHIR specification used to describe the transformation logic from one data structure (data model, directed acyclic graph) to another. It can handle transformations that involve both structural and content/format changes between source and target structures.
In order to execute the mapping instructions expressed in the FHIR Mapping Language, you need to first compile it to an instance of a FHIR StructureMap resource. To do this, you can use the $convert operation on a FHIR server that supports the FHIR Mapping Language or use the HAPI FHIR library directly.
I've built this online tool to provide a hassle-free alternative to the above steps. You can use it to easily compile your FHIR Mapping Language code and pick up the resulting StructureMap JSON, which you can then use to execute the data transformation.
The tool runs the compilation process server-side using the Java HAPI FHIR library.