Rationalism is any statement or decision made on reason. The ability to use the facts at hand and reason about the outcome based on rational thinking. You may also look at empiricism which is based on experience (yours or others) and faith in reaching a decision based on observations, which can be biased, and sometimes in conflict with reasoning or rationality.
It is a conclusion that you come to based on reason, as opposed to something that you accept on faith. For instance, a book might tell you that the earth is round and you accept it on faith; on the other hand, if you got into a plane and flew around the earth and arrived where you started who could rationalise and arrive at the same conclusion.