Truth is the correspondence between and one's beliefs about reality and reality.
Alfred Tarski defined truth in terms of an infinite family of sentences such as:
"The sentence 'snow is white' is true if and only if snow is white."
To understand whether a belief is true, we need (only) to understand what possible states of the world would make it true or false, and then ask directly about the world.
'Truth' is a very simple concept, understood perfectly well by three-year-olds, but often made unnecessarily complicated by adults.