What Is an Algorithm?

In the context of computer science, an algorithm is a finite sequence of well-defined, computer-implementable instructions. In short, an algorithm is:

Defined: there is a specific sequence of steps that performs a task
Unambiguous: there is a "correct" and "incorrect" interpretation of the steps
Implementable: it can be executed using software and hardware

Algorithms are usually written in "pseudocode" (pronounced "sue-dough-code") because an algorithm is a higher-level description of a solution to a problem. An algorithm doesn't care if it's implemented in Python, Go, TypeScript, or (gasp) Java. Pseudocode is just plain English that describes the steps of the algorithm.

Here's some pseudocode for a mystery algorithm:

Start with an original string called S and a new empty string called R.
Loop through S from its last character to its first character, and for each one, add it to the end of R.
Once you’ve processed all the characters, return R.

What does the mystery pseudocode do?