I am going through Warshall and Floyd algorithms and I read that Floyd's algorithm does not hold if a graph has negative circle. I have not been able to fully figure out what  negative circle means. Therefore, I would like to know  more about positive and negative circles. 
What are positive and negative circles in weighted digraph?
graphs
A circle is a path in the graph ($v\to u_1 \to u_2 \to \cdots \to v$) starting from some node $v$, that goes back to node $v$, thus closing a circle.The value of a circle, is the sum of the weights of the edges in the circle. If they sum to a negative value -- the circle is negative; otherwise, it is positive.