AS Physics question about interference and coherent

[MaryaGD]

I am confused about the knowledge interference and coherent. These terms appear in the syllabus~
Anyone help to explain..? And the conditions required if two-source interference fringes are to be observed?

 

[PlanetMaster]

This has one of the best illustrations I've seen: http://www.s-cool.co.uk/alevel/physics/ ... ition.html