Problem Statement:Given a graph with V nodes and E edges, we must find out whether it is connected or not.
From wikipedia: "A graph is connected when there is a path between every pair of vertices. In a connected graph, there are no unreachable vertices. A graph that is not connected is disconnected. A graph G is said to be disconnected if there exist two nodes in G such that no path in G has those nodes as endpoints."
Link To a Broader Description of Graph Connectivity
If you feel more comfortable learning the concept from a video, you can watch this youtube video: