When a tab or range of cells is protected on Google Sheets, by default only the person who created the sheet (the Owner) can edit it.
This is useful if you want to share a spreadsheet with someone, but only want them to edit certain tabs or cells.
You can add protected ranges and sheets from the "Data" menu by selecting "Protected sheets and ranges".
When a range is protected, a person seems to have editing access, but all the tools for editing are greyed out in the ribbon, and they aren't able to change or enter text in cells.
Ranges can have custom permissions. From the Protected sheets and ranges, choose "Set Permissions" and then select "Custom". From this area, you can choose who may edit this protected range. Check the box next to a person with access to the sheet to allow them to edit this range.