If you do not want others to view the content in your Excel sheets. For example the contents is confidential and only you can view it, you can then set a password for it.
To set password for excel file, do the followings:
1. Open the Excel workbook file.
2. Enter your contents.
3. To save, click File -> Save
4. Next select Tools -> Options
5. Then a small window named Options will open
6. Choose tap Security.
7. Enter your password in the Document password field.
8. Enter your password again in the Confirm password field.
9. Click OK.
10. Password has been set to Excel Workbook.
The next time, someone tries to open the file, they will get this prompt.
Now to access the content simply put in your password.