Rationality is the ability to do well on hard decision problems.
Another central theme of rationality is truth-seeking. Truth-seeking is often used as an aid to decision-making: if you're trying to decide whether to get a cryonics policy, you might want to find out whether the technology has any good evidence suggesting that it might work. We can make good decisions by getting an accurate estimate of the relevant facts and parameters, and then choosing the best option according to our understanding of things; if our understanding is more accurate, this will tend to work better.