Definition of connected graph The definition of a connected graph states that:
A graph $G$ is called connected provided for each pair $a,b$ with $a\neq b$ of vertices $\exists$ a walk joining a and b.(equivalently a chain joining $a$ and $b$)...  

What does the definition mean by (equivalently a chain joining $a$ and $b$) .Please help...
A chain is simply a sequence of edges, forming a path. 

A path is a walk without repeated vertices. 

If there is a walk between two vertices a and b, there is also a path connecting them.
An undirected 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. An undirected 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.
A graph with just one vertex is connected. An edgeless graph with two or more vertices is disconnected