How to add edge labels in Graphviz?

You use the label property attached to the edge.

digraph G {
 a -> b [ label="a to b" ];
 b -> c [ label="another label"];
}

The above generates a graph that looks something like this.

@Andrew Walker has given a great answer!

It's also worth being aware of the labeltooltip attribute. This allows an additional string to be attached to the label of an edge. This is easier for a user than the tooltip attribute, as it can be fiddly to hover directly on an edge. The syntax is as follows:

digraph G {
 a -> b [label="  a to b" labeltooltip="this is a tooltip"];
 b -> c [label="  another label" ];
}

Which gives the following result:

You can use label="\E" It will generate bye default label.

For Example:

digraph G {
 a -> b [ label="\E" ];
 b -> c [ label="\E"];
}