Two sources are said to be coherent if they have exactly same frequency, and have zero or constant phase difference. Most of the light sources around us - lamp, sun, candle etc are combination of multitude of incoherent sources of light. Laser is a coherent source i.e. constituent multiple sources inside the laser are phase-locked.
You need coherent sources of light in order to observe effects of certain optical phenomena like Interference in lab. Two parallel slits lighted by a laser beam behind can be said to be two coherent point sources.
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