There is a very good website that explains the Great Vowel Shift (with audio). Not as entertaining as my after dinner party trick, but it does a great job.
My shorter, highly simplified (and rather boring, presented this way) explanation if you aren’t interested in the bells and whistles:
Sometime around 1400 the long vowels of English began to shift upward, which means basically that the degree of opening of the mouth narrowed. Means nothing to you, right? Never mind. If you speak American English, say the word father. That first vowel is an open vowel. Probably the most open vowel in your personal phonemic inventory. Now say the word ink. That is a close (or closed) vowel. If you say the vowel in father and the vowel in ink, you’ve got the extreme of open and closed for most varieties of American English (for the back vowels; there are also front vowels).
Backing up: about 1400 the long vowels began to shift unless they were followed by two distinct consonants. So the the vowel in the world house changed, but the vowel in the word husband did not; once both were pronounced with a sound you might write as ooo. Lots of pairs like this: goose/gosling, wife/midwivery. (Note we are not talking about spelling, but the sounds of the words as they are spoken.
Note: language change is always happening. It’s a natural process, and nothing to get upset over. The Great Vowel Shift was a series of language shifts that took place over a very long period of time — and in fact, never quite finished or caught on in some places. A good many varieties of English spoken in northern Great Britain never participated in the GVS, so in Scotland you are likely to hear hoose for house. The GVS is one of the reasons that written English is so strange; the spellings we use today were set down before the GVS, and for the most part, we never updated our orthography. You see this in many other parts of the language beyond the GVS. For example, the word night. Before 1400 (and still today in some parts of Scotland), this word would be pronounced neeeecht, where the ch is a very throaty fricative. We still write the fricative (night) though it passed out of the spoken language for most of the English speaking world a long timea go.
That’s it, in a nutshell. I really do suggest the other website, the audio examples make it all much clearer.