As humans, we are used to using infix notation to write mathematic expressions:
$$ y = 5 * (x+2)^2 $$
Unfortunately, this notation is very hard for a computer to understand and parse.
Revere polish notation (RPN) can linearize a tree data-structure, which can have memory benefits.
The Hewlett-Packard HP-48 graphic calculator presented RPN to the user (no infix notation was allowed!).
Sequence of Events
Lexer: Converts the grammer (which is the input, i.e. mathematical expression as a string) into a sequence of tokens. Less formally, it can also be called a tokenizer.
Parser: Reads the tokens outputted from the lexer and parses them into a structure such as RPN or AST. It reshapes the tokens into a structure according to rules such as operator preceedance.
Evaluator: Processes the output of the parser, and produces the result the user wants to see. In the case of an AST, the evaluator will typically perform a depth-first evaluation of all the nodes in the AST (which is easily performed with recursion).
For a detailed code-based walk-through of an AST expression parser in C#, see https://mariusbancila.ro/blog/2009/02/03/evaluating-expressions-part-1/.