I don't think it is the fundamental statement of Tarski's theory, but nevertheless an important one.
The kind of truth that applies to sentences, but not the truth as in "he is a true friend".
In this context there is no formal definition of property and there doesn't need to be. The emphasis is "...is a property of sentences". There are other possible choices, for example truth could be a property of beliefs, or propositions, or thoughts, or ideas, or... In Tarski's theory sentences are the truthbearers.
You can learn how to do something, and also that something is true/false. Are you referring to this second kind of learning?
Belief Revision is a model of how to update a set of sentences that are believed by a rational agent, if a new sentence "is learned". But this only formalises learning insofar it says what is learned, but not how and when and why. Regarding your last question, no, a theory of truth won't help you in understanding difficult concepts better. Understanding a concept is very different from knowing its truth condition.