At the French palace, Princess Catherine asks her lady-in-waiting (Alice) to give her an English lesson. (After all, it seems likely that Catherine will be married off to King Henry.)
Alice teaches Catherine the words for various body parts: "la main" (hand), "les doigts" (fingers), "les ongles" (nails), "le bras" (arms), "le coude" (elbow). "le col" (neck), "le menton" (chin), and so on.
When Catherine learns the English words for "le pied" (foot) and "de cown" (gown), she protests that they sound like dirty words and says she would never say them out loud in front of a French gentleman.
Shakespeare's big joke is that, when Catherine explains why these words sound so vulgar, she does say them out loud (several times). With her heavy French accent, Catherine makes the English word "foot" sound a lot like the French word "foutre" (which translates to the f-bomb in English). She also makes the English word "gown" sound a lot like the French word "con" (an offensive slang word for female genitalia).