This question is asked here repeatedly- have you done any searching?
Coherence is a statistical concept that measures how well you can predict results at other times (temporal coherence) and places (spatial coherence), given a single measurement at your current time and place. The 'mutual coherence function' is the most general expression, 'coherence time' and 'coherence area' are restricted to either the time or space domains. Coherence time depends on the (temporal) frequency bandwidth of the source, coherence area depends on the spatial extent of the source.
Well,I guess I understand that.I'm just confused because people say two waves can interfere if they are coherent with each other or sth like this.But I don't understand why.Interference seems to be a local phenomenon i.e. the amplitude and phase of the waves at one point and only at that point affect the result of their interference but coherence is about the connection of two points,in space or time!
The simplest coherence relationships occur when all of the waves are traveling in the same direction, and are in phase ... just like a platoon of soldiers marching together. Incoherence is a mob running in all directions.
Laser beams have very strong temporal coherence due to the stimulated emission properties: in phase, same energy, and same direction. Light from very distant sources is also coherent: from stars or the sun. This is because all of the incoherence is lost along the way ... imagine an alley at some distance from the rapidly dispersing mob: those who arrive there will all be going in the same direction!