Infallible means not capable of being wrong or making mistakes. Although, the word infallible is used correctly in this sentence, the church does not consider the pope infallible in all matters.
It only considers the pope infallible while teaching officially on a matter of faith or morals. The catholic churchs teaching on papal infallibility is one which is generally misunderstood by those outside the church. people often think infallibility means that popes are given some special grace that allows them to teach positively whatever truths need to be known, but that is not quite correct, either. Infallibility is not a substitute for theological study on the part of the pope.