Locks keep the things you've set exactly where you put them.
To lock a day or a meal, open ☰ › Edit schedule (or Preview and confirm in your weekly review) and pick the day.
Locked day
Then tap the padlock on the Day targets card. That day's calories and macros stay fixed, whatever you change in its schedule. Locked days show a padlock in the week strip too. You can still add a workout or change your steps on a locked day, but its target won't move.
Locked meal
Then tap the padlock under the meal's macros. That meal's time and macros stay fixed. The app never moves or changes it.
Typing a target doesn't lock the day
The first time you type a day target on an unlocked day, a note reminds you that day targets follow your activity. If you change a workout or your steps later, the app recalculates that day. To keep what you typed, lock the day.
In the weekly review
Locked days can stop your week reaching the number you chose. See Locked days in your weekly review.