Understanding Djikstra's algorithm

So on CS final, there's most likely going to a question about writing Dijkstra's algorithm on it. I understand what it does, and what it looks like when its run, but I don't know is how to code it, and the purpose of every line of code in it.

Psuedo code given to me by my professor:



I literally have no clue what the purpose of every line of code. What does each line of code do, why does it need to be that way, and how would I implement it in actual code (more specially, how would I set previous to undefined?). What line of code should be at line 24?