Applying sheet protection stops your sheet structure from being accidentally changed by you, or someone else.
You can add sheet protection by right clicking the sheet tab and clicking Protect Sheet or using the Protect Sheet option in the Review tab.
You typically use a password when applying protection, but you can also leave the password box blank.
This still applies sheet protection and stops accidental changes, but allows you to easily unprotect the sheet; make a change and then re-protect your sheet without a password.
This stops accidental changes but allows for quick changes and you don’t have to remember a password.