The input type (often called domain by programmers) of a function is the type of data that it accepts as input.

The output type (often called range) of a function is the type of data that it reports as output.

Selection means deciding (selecting) which part of an algorithm to run based on whether a condition is true or false.

A nested conditional statement is an if or if else statement inside the else part of another if else statement. If the predicate of the outer if else statement is false, then inner (nested) conditional statement will test its predicate and decide what to do.

A string is an ordered sequence of characters.

You've already learned the word index for the position of an element in a list. The same word is used for the position of a character in a string (for example, a letter in a word). So the first input to letter is the index.