It is a document that you are issued with after granting you your residence permit.
A residence card, during its validity, confirms your identity throughout your stay in Poland.
Together with a valid travel document (passport), it entitles you to multiple crossing the Polish border.
You will be issued your residence card after you pay the stamp duty of 50PLN (25PLN in a case of a minor and a student), and submission of registered residence document, or a statement about residence card without an address stated on it (download it here)