And Frenssh she spak ful faire and fetisly,
After the scole of Stratford atte Bowe,
For Frenssh of Parys was to hire unknowe.
(= and French she spoke very well and carefully / After the school of Stratford-at-Bow / For the French of Paris was unknown to her)
Speaking French was a big deal in Chaucer’s England. It was the language of government up until about 1300, and was still the language of law and culture in Chaucer’s time.
The Prioress spoke the French she learnt at Bow (in East London) rather than Parisian French which was apparently different.
Again, this has been argued about, but the probability is that Chaucer (a fluent French speaker who’d been a prisoner of war there) is just having a gentle joke about her French— she can speak it , but not the ‘posh’ kind.
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