The algebraic sum of all voltages around any closed loop in a circuit is equal to zero.
In other words, the total rise in potential (voltage sources) in a loop is exactly equal to the total drop in potential (across resistors, capacitors, etc.) in that same loop.
Mathematical Form
\(\sum V = 0\)
\(V_1 + V_2 + V_3 + \cdots + V_n = 0\)