Starting about 1350, pronunciation of English started to change. Not all pronunciation; mostly the long vowels that were stressed in the word. Pronunciation of vowel sounds depends on the relative positioning of the tongue and lips and palate (remember, I am simplifying). To put it another way: how your mouth forms the space in which the sound resonates determines pronunciation of the vowel sound. What happened during the Great Vowel Shift is that the pronunciation of those vowels moved upwards and backwards in the speaker's throat.
What did this sound like? Without teaching you the International Phonetic Alphabet*, we will try a few examples. The Modern English name would have been pronounced by Chaucer to sound like "na-ma" and by Shakespeare as "neem"; Modern English root would have been "ro-ta" to Chaucer and "rowt" to Shakespeare.
There were exceptions. For instance, "ea" took a different path, depending on the consonants around it. It was long, but it shortened when followed by consonants such as "d" and "th"; so we have "ea" sound like short "e" when "ea" shows up in Modern English dead, head, breath and wealth instead (<—there it is again) of sounding "longer" as in great and break.
Consonants stayed the same, although "silent letters" did develop later. Chaucer would have pronounced "knife" something like "ka-nife"; that is, both consonants would have been pronounced; it was later that we got lazy and stopped bothering with the "k" in "knife" and "knowledge."
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The sad part is that England had become a literate culture before the GVS was done. Printing was standardizing spelling even as pronunciation was going through its evolution. Therefore, the pronunciation of words moved well beyond their original spelling, creating issues for schoolchildren and non-native speakers for centuries to come.
*Which, to be honest, would require me to learn it first.