why |account| for a find method?

There are many accounts and they are all stored together in the @accounts instance variable. This method goes through them, checks each account (ONE AT A TIME!) to see if that accoun't full_name parameter is equal to the first_name + last_name of the person we are looking for. This |account| construct is very important and you will see it a lot in Ruby (and later, in Rails etc.). When the method is going through each account individually, it needs to stores them temporarily somewhere, in the "account" local variable in this case. So it does something like this:

Take the first account from the whole @accounts set, put this single account temporarily in the "account" variable, see if that account matches the query. If it doesn't match, take the second account from the @accounts and put it temporarily in the "account" variable again (overwriting the previous one, since it wasn't the one we were looking for), see if this one matches. If not, take the third, put it in the "account" variable etc...

It doesn't need to be named "account", this could as well be:

@accounts.find{|x| x.full_name == "#{first_name} #{last_name}"

"Go through each account and use the x variable to store each of them temporarily while doing comparisons"

You just need to match the thing between the || symbols and the thing before the .full_name. And it would do the same thing. It's a convention to use the singular form of the noun, to make it more readable and intuitive.

Hope this description helps.