Rationality just means using logic to arrive at truth. Rather than using emotions / cognitive biases.
Logic simply involves correctly interpreting available information. What does it mean to be correct / incorrect? Correct interpretations cannot be disproved by existing information, while incorrect ones can be disproved by simply examining available evidence.
And because new information can potentially disprove old interpretations, correctness is always relative to existing information.
Rationality is about making the most out of available information. It does not guarantee truth, because as long as our information is limited (i.e. as long as we’re human), there is always the likelihood that new information will radically change our previous interpretations.
“Real knowledge is to know the extent of one’s ignorance.”—Confucius