It’s not possible to password-protect the entire iExplorer app, but you can password protect (encrypt) your backups in iExplorer. This can’t be done to an unencrypted existing backup after it’s been made, but you can enable encryption in iTunes for each of your iDevices and then make a new backup for each one that requires a password.
The Backup section of iExplorer is displaying the data within an iTunes backup on your computer. When you choose to Back Up Now, iExplorer creates a new iTunes backup for you and then displays the data from within the iTunes backup in the iExplorer window. You can protect your backups with a password in iTunes on the Summary tab for your device. Here’s an AppleCare article for how to encrypt your backup with a password and one of our articles about accessing encrypted iTunes backups in iExplorer: