It is understanding how the Great Vowel Shift moved pronunciation away from spelling — and how that older spelling was fixed and fossilized by the 15th-century introduction of printing.
Three quick items:
2. A website devoted to the Great Vowel Shift, with discussion of Chaucer, Shakespeare, etc. Check out the dialogs (top of page), particularly the one for Middle English — pre-Vowel Shift.
3. The technology of printing came to England from countries that did not use particular English letters such as thorn (Þ) and edth (ð) — and several others as well. Compromises were made, and some are still confusing people.
The lingering offender is thorn. If you read this, you can now sneer at anyone who pronounces “ye olde” with a Y sound as being inadequately educated. (You can also sneer at anyone who names a business, etc. Ye Olde Whatever in the first place, on general principles.)
3.5. You will also understand the Scottish pronunciation of the name Menzies.