Would someone be as kind to tell me what is meant in the following notation by the subscript 'thingies' that look exponents but are displayed below the number?

I'm looking for a formal title so I can google up on the definition, thanks for your help. More generally, what are good resources for looking up idiosyncrasies in notation?

Edit: Nevermind I just realized that the numerals are denoting the index / position in a vector / set. But where would I go in the future when notation confuses me?

Edit 2: ok take this for instance: We can approximate this value by taking a point somewhere near to P(x, f(x)), say Q(x + h, f(x + h)).

From:

3. The Derivative from First Principles
I don't understand the P(x, f(x)) part specifically, are they denoting probabilistic distribution, I don't think so, I've never seen a point defined like that when I've worked with Highschool level euclidean geometry informally, where do I even start to begin learning these counter-intuitive conventions