Every modern language seems to have its vital, foundational literary work: Italian has Dante's Divine Comedy, Spanish has Don Quixote, and English has Chaucer's Canterbury Tales. And yet, like other such works, the writings of Chaucer are more often talked about than read; unlike Shakespeare's, his characters have not so often strutted upon the stage. In the UK, the BBC has done them both as a period puppet piece as well as a modernized version, and in 1972 the great Italian director Pier Paolo Pasolini made a memorable film version -- but here in the US there have been no major film or television adaptations, unless you count the somewhat squishy "A Knight's Tale." Still, Chaucer's influence has been deeply felt; his Troylus and Criseyde was one of Shakespeare's sources for his play of the same name; the Wife of Bath's Prologue was translated in 1700 by John Dryden; and in the twentieth century there have been no fewer than seven translations or adaptations into modern English, most recently by Peter Ackroyd (in prose) and Sheila Fisher (in verse).
And Chaucer's language is very close to ours -- close enough that, with a modest amount of practice, we can pronounce it and understand it reasonably well. The main differences are that many combination of letters which are now silent, such as kn and gh, were pronounced, as was the e on the end of so many words which has since become our modern English friend "silent e." Lastly, the vowels of Chaucer's day were further back in the mouth, since the "Great Vowel Shift" which moved them forwards had not yet taken place. Thus Chaucer's "Whan that Aprille with his shoures sote" is pronounced as "Whahnn that ah-prill, weeth hees shooriss soat-uh" in our reconstructed Chaucerian mode.
The other difference is mainly vocabulary -- "lexical" as linguists would say. Chaucer still used a number of words left over from older English; a "swevene" was a dream; "eke" meant "indeed" or "as well"; he used "lykned" instead of "likened," "hem" instead of "them" and "hire" instead of "their" there was no neuter possessive "its" so everything had to be "hys" or hers. Happily, there are many fewer such survivals in his verse than in that of the Gawain poet, who, living the north-west Midlands, was removed from many of the changes of London English and still used athele (noble), sithen (since), schawe (show), and lovelokkest (loveliest).
Chaucer was also on the forefront of many changes of which modern English has been the beneficiary; he introduced many words and usages from French, some in their Anglo-Norman senses, nearly all of which are still with us today: bachelor, melody, adversary, bounty, refute, vein, army, season, and devout. He borrowed a few Latin terms, though most often via their French versions, and his verse structures, such as "rhyme royal,"turned out to be particularly effective in English.