A graph comprises of nodes and edges. The nodes can connect to another node when an edge connects the two. The edge can be directed or directional in that it describes a starting node and an ending node. The graph with nodes and directed edges is called a directed graph.
When a directed graph has a node with no outgoing edges, perhaps it has only other nodes connected to it via directed edges, the node can be said to be a graph sink.
An example is the following graph:
A -> B
A -> C
B -> C
The node C has only ingoing edges with no outgoing edges.