How does "or" work and what does it do (and does it not do some things i ask)?

I can't find anything on google so i've resorted to here. When I use or does it randomly select a value or does it choose the best option. For example, if i have abc and abcd but abc doesn't exist, will it choose abcd like this?

local abcd = script.Parent
local v = abc or abcd
print(v)