Arildno, there are an awful lot of people here for whom English is a difficult language! Cut them a little slack.
teng125, and alphabetical character (to make arildno happy) with a subscript n, normally, is just a way 9f distinguishing among many different values. One application is with vectors where we may label the "x","y", and "z" components of the vector a by a1, a2, a3.
Another important application, which is likely to be what you are looking at, is to label numbers in sequences. The only difference between a sequence and a function is that the "domain" of a sequence is the positive integers: 1,2, 3, etc. A standard way to distinguish them is to use x for a "real variable" and n for a "positive integer variable". Some texts actually use a(n) for a sequence (more correctly the nth term in the sequence) but it is more traditional to use a subscript: an.