The basic idea is that you can collate some of the myriad accounts you follow by creating lists – and it’s entirely up to you how you group them and what the list is called. Then by navigating to the list you may either see a filtered feed of the tweets from that select bunch of accounts, (click on ‘Tweets’), view the ‘List Members’ or indeed see if any ‘Subscribers’ have taken an interest in the list you created.
Here for instance is one of mine called Nature, https://twitter.com/CarrsWetland/lists/nature created a while ago. You will see it has 108 members – these are accounts I chose to add to my Nature list. (You can do this by the way by clicking the settings drop down beside any user profile and selecting ‘Add or remove from lists’ then you either nominate an existing list you have set up previously or create a new list.)