What does d mean on Polish visa?

D refers to D visa, which is generally a student visa.

Polish National Visa (“D”)

This type of visa entitles the holder to enter Poland*** and stay in Poland*** and the country without interruption or the right to stay in the Schengen Area multiple times for a total stay of more than three months within a period of not more than one year during the validity period of the visa. National visas issued by Schengen countries also entitle the holder to travel to other Schengen countries for three months within six months.

Poland changed its policy in May last year. In the past, the D visa was for one year, but now you can apply for a maximum of 15 months, in order to prevent you from failing the exam and making up for it.

http://pekin.msz.gov.pl/zh/sprawy_konsularne/general_visa_information/;jsessionid=E47B4A925A1AA16BEF97A8C375651EA8.cmsap5p