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Protecting the sheet means that the content of cells cannot be changed and/or formulas can be hidden from view.

To protect a sheet:

From the Tools menu, select Protection, and then Protect Sheet.
To protect the cells in the sheet using the Protect Sheet option:
1. Press Ctrl+1, and select the Protection tab in the Format Cells dialog box.
2. Select the Locked checkbox and click OK.
3. Protect the sheet (see above). To protect formulas in cells.

To hide text/formula in the Formula bar:

1. Press Ctrl+1, and select the Protection tab in the Format Cells dialog box.
2. Select the Hidden checkbox and click OK.
3. Protect the sheet (see above).

The new Protect Sheet dialog box in Excel 2002 and Excel 2003 provides various options for protecting and unprotecting different elements in the sheet.
Screenshot // Protecting the Sheet or the Cells in the Sheet