What is the meaning of the name Justina in Poland?

It's just a name. As in every language where it exists (English, German and French - Justine, Italian - Giustina) it comes from the Latin name Iustina (feminine of Iustinus, meaning "fair" or "just".)

In Polish, the correct spelling of this name is Justyna (not Justina), pronounced "Yoosteena".