Your data (weights and notes) for each exercise will be saved on the app when you complete your workout. If you decide to go back and restart the same workout to complete it again, your data will still be there; and you can save new weights and notes for each exercise every time you go through it.
Once you've completed an exercise multiple times, going back to it and clicking on "Details" will take you to a page where your set history is available - each entry you make will be listed there with each own date/time stamp so you can track your progress.