From the webpage:
Source code analysis is full of graphs: abstract syntax trees, control flow graphs, call graphs, program dependency graphs and directory structures, to name a few. Joern analyzes a code base using a robust parser for C/C++ and represents the entire code base by one large property graph stored in a Neo4J graph database. This allows code to be mined using complex queries formulated in the graph traversal languages Gremlin and Cypher.
The documentation can be found here
This looks quite useful.
Makes me curious about mapping graphs of different codebases but shared libraries together.