The Bisect command allows you to find out which (past) commit introduced a problematic behavior in your project, e.g., a bug which was not present a half year ago.
Start a Bisect command by using the menu item Branch|Bisect|Start. Typically, the current branch (HEAD) currently shows the bad behavior, so you can select Start Bisect with Bad HEAD. Then the HEAD commit will show up with as red dot which means bad (good commits will be shown as green dots).
Now you will have to find a good commit in the history of HEAD that worked fine. To do that, right-click commits in the history of HEAD and invoke Check Out. Now test your application whether it runs fine or not. Select the corresponding buttons Mark HEAD as Bad or Mark HEAD as Good (from the banner). If your application still shows bad behavior, you need to go back even further and try further commits. Once you have marked a bad and a good commit, the bisecting process starts.
Git will check out a middle commit between the closest bad and good commits. Verify whether your application behaves correctly or not and use the buttons Mark HEAD as Bad or Mark HEAD as Good again to tell Git about it. This will use a binary search to quickly find the problematic commit that causes the trouble.
You can always abort the bisecting-mode by clicking Abort (in the banner). This will check out the branch which was checked out when starting the Bisect command.