Backtrack is nothing but how many tries(attempts) have done inorder to generate the given string from Start symbol by the Top down parser.
Intially,from start symbol S first production is selected follwed by production of B but with the first production selected we couldn't generate the given string aab
S->aB->aaB->aa // Unsuccessful
Backtrack
Next it will select second production from start symbol S and first production from A
S->aAb->abAbb->ababb //Unsuccessful
Backtrack
Now it will select Second production from start symbol S and second produciton from A
S->aAb->aab //successful , no backtrack
Thus 2,backtracks are required to generate string aab