How it works?

Set dist[source] = 0 and every other vertex to a large "infinity". Then relax all edges, and repeat.

Step-by-Step Process:

1. Make one pass: go through every edge and relax it.
2. Repeat the pass V - 1 times. Why V-1? A shortest path visits at most V vertices, so it has at most V-1 edges, and each pass locks in at least one more edge of every shortest path.
3. Any vertex still at infinity is unreachable (report -1).

A single pass usually is not enough, because an edge relaxed late may depend on one relaxed even later. Repeating V-1 times guarantees every shortest path is found (when there is no negative cycle).