Rationality is a core skill that any value investor should possess. If you are someone who is looking to buy assets when their owners are panic selling because you believe them to be undervalued, then you - by definition - have to be a very rational person. This type of contrarian behavior can only result in success if you are able to really cut through the emotion of the situation and drill down to the core value proposition of an investment.
Munger's advice is to try and avoid confirmation bias. This bias occurs when investors go looking for evidence that supports a belief that they already hold, rather than looking for evidence that challenges that belief.