A belief is the acceptance of a proposition. A true belief is one that has been examined by the believer and remains a belief. We often accept propositions, only to discover later that we were wrong. Being wrong can be the result of many things: lack of other knowledge that would have caused disbelief; a persuasive argument that you later reject; the proposition was rational-sounding but it was a fallacy.
A rational proposition that is not a fallacy has justification, that is, it is 'justified'. This means the logic is sound and it has a correspondence to facts of reality. (See 'correspondence truth').
Therefore, a 'justified true belief' is one that has been shown to be logically sound, or is accepted as logically sound.
It may or may not be 'defeasible', in other words, defeatable, by a better argument. The Copernican Revolution was the defeat of Catholic justified true belief, by the arguments of Galileo who used the mathematics of Copernicus. (See 'defeator arguments' or 'defeasors')
President Kennedy had a justified true belief that we could get to the moon, because he was shown the proof, without which his belief could not have been justified.