The reading page is the list of friends and communities you follow (are subscribed to or have joined). The network page is the list of your friends' friends and the communities they follow or have joined. It's a nifty way to see other communities and friends. The other way is to look at a person or community's profile page and see what they have subscribed to and/or joined. The caveat with using a profile page as a basis for what you might join or subscribe to is that some people (myself included) wind up with a lot of accounts listed on their profile page - some of which aren't even active anymore - and it can get messy. Using the Network tab gets you a quicker, more up-to-date snapshot of what your friends of friends are into. Have fun exploring!