That's a huge subject. Books have been written about the GVS. Opinions about what happened and when, over what period, and with what effects on communication, differ widely (and sometimes wildly). Start here: Great Vowel Shift - Wikipedia, the free encyclopedia
In principle, the point of articulation of a lot of English vowels changed, with resulting moves throughout the vowel system, often causing homonymic clashes (puns). In some cases there was enough contextual support for such a clash not to matter, and two meanings/pronunciations for a single spelling have survived; in other cases, one 'won'. For example let meaning 'allow' pushed out let (with a different 'e-vowel') meaning 'obstacle'. The latter meaning is now relegated to tennis, and to the phrase 'let or hindrance'.