File encryption is additional security you can apply to a workbook. File encryption scrambles your password to protect your workbook from unauthorized people from breaking into the file. You can use the Security tab in the Options dialog box to set password protection using file encryption. If you are aware of file encryption types, you can select the one you want to use. You don't have to worry about the encryption, Excel handles everything. All you need to do is remember the password.
Apply File Encryption