The name means small chestnuts, but I have no idea why. They don't look much like real chestnuts, neither taste like them.
Kasztanki are coated with dark chocolate and have crispy cocoa-flavored stuffing with crushed wafers. They were introduced during communism by Wawel, one of the oldest Polish confectionery plant. Kasztanki quickly became very popular and they still maintain their popularity. I recommend buying a box of Kasztanki as a souvenir from your visit in Poland.
Wawel also makes a chocolate bar called Złoty Kasztanek (golden chestnut) that has similar taste to Kasztanki pralines. Personally, I prefer pralines because they contain more stuffing as compared to chocolate bar version.
Price (June 2012): around 12 PLN (3 EUR) for a 430g box