Really Basic S3 Upload credentials

I'm giving Amazon Web Services a try for the first time and getting stuck on understanding the credentials process.

From a tutorial from awsblog.com, I gather that I can upload a file to one of my AWS "buckets" as follows:

s3 = Aws::S3::Resource.new

s3.bucket('bucket-name').object('key').upload_file('/source/file/path')

In the above circumstance, I'm assuming he's using the default credentials (as described here in the documentation), where he's using particular environment variables to store the access key and secret or something like that. (If that's not the right idea, feel free to set me straight.)

The thing I'm having a hard time understanding is the meaning behind the .object('key'). What is this? I've generated a bucket easily enough but is it supposed to have a specific key? If so, how to I create it? If not, what is supposed to go into .object()?

I figure this MUST be out there somewhere but I haven't been able to get it (maybe I'm misreading the documentation). Thanks to anyone who gives me some direction here.