"A car" is the one that the listener has no idea about beca use it is preceded by "a" which is indefinite article. The car _ is known by the listener because it is preceded by "the" definite article. This car_ is also preceded by "this" which is definite article that make the known known by the listener and the speaker.

Hier, j'ai acheté une voiture. Hier, j'ai acheté la voiture. Hier, j'ai acheté celle voiture-ci.

j'ai acheté une voiture j'ai acheté la voiture j'ai acheté cette voiture

Remember that "explain X" means that X is a mystery. You don't understand X: "explain these sentences"; "explain what happened"; "explain your reasons." Now, imagine what "explain me" really means in English. :) (Tip: the phrase is "explain to me", or "explain this to me". In fact, your question should be, "Could you explain this to me, please?")

j'ai acheté une voiture j'ai acheté la voiture j'ai acheté cette voiture