What does the arrow with 2 lines through it mean?
That is the page or scroll up key. On a MacBook the keystroke is Fn-Up Arrow.

“What Do the ⌘, ⇧, ⌥, ⌃, ⎋, ↩, ⇥, ↖ and ↘ Menu Symbols Mean?”

Each symbol corresponds to a key on your keyboard:

⌘ — Command/Apple key

⇧ — Shift key

⌥ — Option/Alt key

⌃ — Control key

↩ — Return key

⇥ — Tab key

⎋ — Escape key

↖ — Home key (Function-Left Arrow on laptops)

↘ — End key (Function-Right Arrow on laptops)

There is a very handy reference at harvard.edu.