Epistemology is the study of how people should form beliefs and credences about the nature of the world. Beliefs and credences are purely evaluative attitudes: they are simply about the way that we think the world is. A person might believe that it will rain, for example, even though they hope that it will not.
Beliefs are all-or-nothing attitudes: we either believe that it will rain or we don't believe that it will rain. Credences, on the other hand, reflect how likely we think it is that something is true, expressed as a real number between 0 and 1. For example, we might think that there is a 80% chance that it will rain, and therefore have a credence of 0.8 that it will rain.
It is widely held that beliefs are rational if they are supported by our evidence. And credences are rational if they follow the probability axioms (e.g. a credence should never be greater than 1 in any event) and are revised in accordance with Bayes’ rule.
Chignell, Andrew. 2016. The ethics of belief. In Edward Zalta (ed.), The Stanford Encyclopedia of Philosophy, sect. 4.