It is a representation of an oscillating quantity. In phase space with an angular velocity equal to the angular frequency of the original trigonometric function.
The projection of the phasor onto an axis at a specific time gives the value of the quantity at that time. Phasor diagrams are used in simple harmonic motion and RLC circuits which have elements that are out of phase with one another and thus difficult to work with in configuration space.
Read More: Phasor Representation
Three ways to represent phasors: