Neo4j graph-algo is a component that contains Neo4j implementations of some common algorithms for graphs. Algorithms considered production quality can be found in org.neo4j.graphalgo.GraphAlgoFactory factory. There you can find algos for shortest paths, all paths, all simple paths, Dijkstra and A* etc.
See the documentation for more information.
There are other algorithms which can be used on smaller graphs, f.ex. calculating centrality, betweeness, closeness, eccentrity and more. Those algos aren't designed to scale to very big graphs, but can still be useful in some scenarios. They reside in the org.neo4j.graphalgo.impl.centrality package and are not considered production quality.