A crucial requirement for sustainable interference is coherence, which simply means, waves arriving at a given point from two sources must have a definite phase relationship. Obtaining coherent waves is extremely difficult as light emitted by ordinary sources are not inifinitely long but consists of wave trains of finite length. Basic mechanism of emission of light invloves atoms radiating EM waves. A typical atom radiates for  ${10}^{-9}s$. This time is called coherence time $\left({\tau }_C\right)$ and length of wave train $L=\left(C\right)\left({\tau }_C\right)$  is called coherence length.
So waves emitted by two ordinary sources, reaching at a point can have a constant phase difference for a maximum of  ${10}^{-9}S$ , after which two new waves will reach the point. Situation gets worse if we consider interference of waves emitted at different times, in such cases, actual time for which phase difference between waves remains constant is much less than  ${10}^{-9}S$ . The above mentioned facts can be explained easily with the help of diagram as shown below.