for ... in (meaning)

a for in loop loops over all the values in an array and makes the item accessible in a local variable.

so with

for card in cards 

the card variable is the item in the array, and cards is the array. you can then do something to every single card through the use of the card variable

for example, if I had an array of animals called animals, and wanted to loop over every single animal in that array and print it I would use

for animal in animals {
println(animal)
}

and every item in that array would get printed.

the name you give to the variable that comes after for does not really matter, it can be anything you want. Most of the time you have an array that stores a collection of things, so usually the variable is just the singular form of that thing

for thing in things {}
for rabbit in rabbits{}

but it can be any valid variable name

for asdf in things {}
for blah in rabbits{}