"Position to term rule" is a mathematical rule. I am not exactly sure what you mean by "in Vietnamese". At enotes, the educators answer all the questions in English only, so I will stick to the same.
The position to terms rule is an explicit rule and allows us to determine the nth term of a given sequence (assuming we can find the sequence). for example, if we have a sequence 1,2,3,4,.....we know that the 'nth' term in the series will be given by 'n' (e.g., 5th term is 5, 8th is 8 and so on).
so we can find any term of this sequence.
Similarly for a sequence 1,3,5,7,.......any term is given as An = 2n -1 (e.g., A2 = 2x2-1= 3, etc.). so we can find any term, say 8th term in this sequence would be 2x8 -1 = 15, and so on,
In the position to term rule, we do not need to know the preceding term to know the next one (as in Term to term rule).
Remember not all the series have a sequence that can be identified so easily, say sequence of prime numbers.
Hope this helps.