You may wish to protect single cells, sections of the worksheet or entire worksheets from change or access depending on:-
Protected cells cannot be changed, moved or copied without first being unprotected.
Protection can be secured using passwords.
However care must be taken with passwords. If forgotten, your spreadsheet is irretrievable.
In Excel cells are LOCKED as a default setting.
However this has setting no effect until the worksheet itself is protected.
Properly designed worksheets usually have a combination of protected and unprotected areas.
Labels and formulas are often protected to stop users from accidently deleting or changing them.
To create a worksheet with a mix of protected and unprotected cells you must unlock the cells you wish to be able to change before protecting the worksheet.