In graph theory, a source of a directed graph $D = (V(D), E(D))$ is a vertex of it whose in-degree is zero.
The book CLRS makes these statements:
Given a graph $G = (V, E)$ and a distinguished source vertex $s$, breadth-first search systematically explores the edges of $G$ to “discover” every vertex that is reachable from $s$.
I know this is an amateur question but does source have the same meaning here (at least when the graph is directed) or it's just some word the book uses without any particular reason? Maybe by source it means a root.