What can I searchengine to learn about this? I have no idea what

.pop() and .push(n) are methods. Googling `pop method` or `push method`returns what you probably want. If you dont understand how to google something about programming just add the language into the search and the exact word you dont understand like `javascript prototype`

Its code manipulating a data structure called a 'stack', which is easy to think about if you imagine a stack of real-life flat objects. For instance, fancy plates - you can put them on top of each other (push) but you wouldn't dare try to remove one from the middle. Instead, you always remove the top item (pop). Stacks arent really necessary in code, but they're useful when you want certain structures used AS stacks to be clear to yourself and others.

In your image, the stack would have +1 element if it was empty and pop() doesnt return an error. Otherwise it's +0 elements.

Thank you very much