Protected cells vs locked cells
Protected cells are listed in the cell's information dialog box. You cannot change these cells. You cannot enter data in them or link comments to them. You can only consult them in read-only mode.
The protection cannot be removed from these cells.
Locked cells are listed in the cell's information dialog box. Locking a cell enables you to ensure that the amount contained in the locked cell will not be changed when it is included in a spread calculation.
You can however change an amount in a locked cell by entering another amount in the cell.
Example
The total in cell C3 is 200. The breakdown of this total is 100 in cells C1 and C2 respectively. Lock cell C1. Change the total from 200 to 250. C2 changes to 150 but the amount in cell C1 remains unchanged, i.e. it stays at 100.