Is there a way to secure a Google Sheets?
Your spreadsheets are already all password protected, in that one must log in with their google account (and thus password) to view the spreadsheet. You can control who may access the spreadsheet by clicking on the "share" button in the upper right and editing what permissions users with access to the spreadsheet are given.
Yes, you can use the Google script from here to have a password-protected spreadsheet. It gives you two options - encrypt and decrypt. You can set a password and if the password is wrong, all you will see is obfuscated text.
nhingle's answer is correct, but most people are logged into Google 24 hours a day on phones and PCs. That means if somebody has access to these momentarily, they have full access to Google docs as well.