Poland has both maritime and continental climate because it has influences by both the warm sea air masses and by the colder drier continental air masses.
The northern part of Poland comes out on the Baltic Sea, which is connected through the Kottegat Bay and Skagerrak Strait with the North Sea and further with the Atlantic Ocean. Because of this connection, the influence from the warm and wet Gulf Current manages to get to the northern parts of Poland, but because it is significantly weaker, it only manages to influence that part of the country.
The interior of Poland, on the other hand, is much drier, and it is predominantly under the influence of the continental air masses, which in summer bring in dry and hot weather, while in winter it is very cold and snowy.