In practice
A journal turns experience into data. Recording the setup, entry, stop, target, result and the reason for each trade makes it possible to see which setups make money, at which times, and which mistakes repeat.
Notes on behaviour matter as much as the numbers: whether the plan was followed, and what you were feeling when it was not.
What to watch for
It only helps if every trade goes in, including the embarrassing ones, and if it is actually reviewed. A journal that is filled in and never read changes nothing.