With Auto Scheduling enabled, every task is automatic by default and moves with its dependencies. Sometimes you need a task to stay on a specific date, such as when a contractor has committed to showing up on a certain day.
To lock a task
Find the task in the Scheduler and locate the Auto? column.
Toggle the task from Auto to Locked.
Drag the task bar to the date you want it to start, or set the date in the task's edit window.
The task holds that date even if an earlier task runs late, while the rest of your schedule keeps taking advantage of Auto Scheduling.
You can also set this from the task's edit window under the Auto Scheduling section. To let the task move with its dependencies again, toggle it back from Locked to Auto.
Read the full article: How do I lock a task to a specific date?