As soon as you open an alternative, Flex will download the alternative locally to your computer. This is when the project gets locked for your colleagues.
This means no other colleague can work on the same project while you are editing anything inside the alternative. This ensures that your changes are saved correctly without anyone overriding them.
You can see the user and PC name of where the project is open when you double click to open an alternative.
Should a project be accidentally locked, you can release it inside the Flex platform see article Flex: Unlock a Project and Alternative