If you share a workbook with others and want to stop them changing the sheet cells you can protect the sheet. By default all cells are protected. If you want to allow them to enter information in some cells, but not others, then you need to unlock the cells to allow them to be changed.
Select the cells, go to format cells, (right click on the cell and choose the Protection Tab), and untick the Locked.
Or you can choose Home Menu, Format, Lock Cell. The Lock Cell option here is actually a toggle and will unlock a locked cell, and lock an unlocked cell. The background change shows you which you will do be clicking on it. If the cell is currently locked the option will show with the box around the padlock, so clicking will unlock the cell.
Locking a cell only has an effect once you Protect the Sheet.
When you Protect the sheet, you can’t change a locked cell.
When you select Protect Sheet, you actually get a large number of options about how many things you want to restrict. The default is too allow Locked cells and unlocked cells to be selected. Remember locked cells won’t be able to be changed but unlocked cells will be. If you untick Select locked cells you won’t even be able to click on those cells.
You can provide a password that would need to be entered to Unprotect the sheet. Don’t lose the password, there is no easy recovery if you do. However, don’t rely on a password to keep information secure. Protecting the sheet doesn’t encrypt information and passwords can be got around.
Depending on what you want the user of the sheet to be able to do, tick the required options and select Ok.