Tower's sidebar lists all of your project's local branches & tags as well as remote branches.
Selecting an item in the sidebar will show its commit history; selecting multiple will show a combined history of the selected items.
Tip: The "/" character in a branch's name can be used to simulate folders. This is a naming convention in Git and is respected in Tower, where such items are displayed as collapsible folders in the sidebar.
If you should not want this behavior, you can deactivate it in Tower's Preferences by unchecking the option "Group refs sidebar".