Question:
Which means i-th, I'm having trouble understanding its use in arrays and arrays.
Answer:
When you refer to index 0 of an array , you can say the first element, right? Just as 1 is second, 2 is third, and so on. The 19 is the twentieth.
How would you talk about the i element of the array ? i-th. So when you need to define a "generic" (variable) position of the array , you can use this expression to indicate which element will depend on the value of i to know the real position.
-th (from the ending of hundredth or thousandth) composition element
It expresses the notion of ordinal and fractional numerals (ex.: cagagéth, nth, infinitesimal).
"-ésimo", in Priberam Dictionary of the Portuguese Language [online], 2008-2013, https://dicionario.priberam.org/-%C3%A9simo [accessed on 12/03/2019].