Update: Ok, I've seen that there is also a number assigned to the feed, but that is still not secure.

If your lists are private, then your password is required to access the feeds and the feeds are only accessible via HTTPS (i.e. the feeds are sent encrypted).

Your lists are automatically private unless you publish them. From the help page:

'Your Atom feed for a list will be accessible by others if you choose to publish that list.'

Some online aggregator services give you the option of making your feeds public on the aggregator's website. If you're using an online aggregator, you should check your settings to make sure that your feeds are not being shown in your profile on their website. I'm not sure if the confusion came from this part of the help page, but I'll try to make things clearer in the help section.

Hope this helps!

Emily,

thanks. I was confused because I could access the feeds without entering a username/password. But I found out that this seems to be related with my session and when using a different, not logged in, browser it doesn't display my items anymore.

Yes, if you're logged in then you'll be able to view in the feeds in your browser.

Thanks for letting me know!