To "freeze" a row is not synonymous with "protecting" a row.
When you Freeze a row, for ex. by dragging the freeze bar between row 3 and 4 in your screenshot, you are locking them in place from a UI perspective. This means that those rows will stay at the top of the screen even as the user scrolls down through the worksheet.
Ex: This is helpful when you want to make sure the column headers are always visible.
When you "Protect" a row, range, or sheet, you are locking users out of editing them without warning or permission.
That page helps describe how to Protect a sheet or a range. in particular:
- Open a spreadsheet in Google Sheets.
- Click Data and then Protected sheets and ranges. A box will open on the right.
- Click Add a sheet or range or click an existing protection to edit it. To protect a range, click Range.
- Range: To change or enter the range you’re protecting, click the spreadsheet icon and highlight the range in the spreadsheet.
- Click Set permissions or Change permissions
From there you can define what permissions you'd like to apply.