A list is a curated group of Twitter accounts.
You can create your own lists or subscribe to lists created by others.
Viewing a list timeline will show you a stream of Tweets from only the accounts on that list.
Within your account, you may wish to create lists for specific areas of your niche.
For example, if your niche is 'Weight loss and Fitness', you may wish to create different lists for "Exercise tips", "low-calorie diets", "fitness market-leaders".
Consider making lists of the top influencers in your niche so you have a well of content to take inspiration from and retweet.
You can find further information on creating and editing lists here.