I do not understand what subscript numbers mean

  subscripts identify different instances of objects that have a similar meaning or definition

Step-by-step explanation:

You apparently are interested in the meaning of the subscript N in the explicit formula ...

Here, it is used to identify different terms of a sequence. When a value is substituted for N, it replaces all instances of N. Using your example, the first three values of are ...

So, if you're asked to find the 20th term of the sequence, you put 20 where N is, then do the arithmetic.

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Subscripts are also used to identify elements of a matrix or coordinates in multiple dimensions. For this purpose, there may be 2 (or more) subscripts, whose order has meaning. The subscripts 24, for example, in refer to row 2, column 4 of a matrix named A. That is, the subscripts are in row, column order.

In short, subscripts are used to identify different instances of similar objects.