Graph density is a measure that indicates how many edges are present in a graph compared to the maximum number of edges possible. It is calculated by dividing the number of edges by the total number of possible edges, which depends on the number of vertices in the graph. A higher density means that the graph is more connected, while a lower density indicates a sparser structure. In a complete graph, every pair of vertices is connected by an edge, resulting in the highest possible density. Conversely, a graph with few edges relative to its vertices has low density. Understanding graph density helps in analyzing the connectivity and structure of networks, such as social networks or transportation systems.