FHIRPath is a language for navigating through FHIR data models and extracting useful data for use and interpretation. It is similar to XPath and JSONPath, but is adapted to work with FHIR data.
It is often required to write FHIRPath expressions as part of a FHIR implementation. To make sure that the FHIRPath expressions are correct, you typically try them out against some FHIR resources, programmatically using a FHIRPath evaluator.
To make it easier to prototype and test FHIRPath expressions, I've built this online FHIRPath Testing Tool. Using this tool, you can easily input your FHIRPath expressions and check if they retrieve the correct data from the FHIR resources.