iExplorer lets you access any encrypted iPhone or iPad backups by entering your encryption password for that device. The encryption password is one you would have chosen when setting up your iPhone, iPad, or iPod Touch to backup in iTunes and is one you would have been prompted to enter twice when you checked the box to Encrypt its backup in iTunes. This password is separate from your Apple ID or iTunes Account password and is not at all related to iExplorer. It's one you would have chosen before even using iExplorer. The Apple article below explains more about your encryption password:
If you have an iOS 10 device and can't decrypt your iTunes backup with a password you know is absolutely the right one, scroll down to the section at the bottom of this article for what to do.
If you don't remember which password you chose for encryption, you should try some of your commonly used passwords to see if you can figure out the correct one. Fortunately, there is no limit to the number of times you can enter incorrect passwords, so you don't need to worry about any consequences if you keep entering the wrong ones (i.e. dataloss or blocked access). You may also want to try some generic passwords such as "1234" or "12345" or "password" in case a salesperson or friend setup the password for you and chose a temporary, easy one.
If you have an iPhone or iPad that is owned by your employer or school, the password was most likely setup and chosen by them.
If you setup your iPhone or iPad on a Mac, the password might be saved in your Mac's Keychain. To find out how to retrieve it, just click HERE.
If you are unable to figure out the correct password after trying all of the ones you know and any generic, obvious passwords, unfortunately, there is no other way to recover your password (unless you wrote it down somewhere). Apple doesn't even have access to it since the password is only stored locally on your iPhone or iPad and on any computer where you've made an encrypted backup. There's also no way to find out a hint for what it might be or any way to reset it.
If you are wondering whether it's possible to just disable encryption and create an unencrypted backup, that would require you to enter the encryption password first anyway and for some company or school owned devices, the checkbox to Encrypt the backup in iTunes is checked and grayed out (can't be unchecked).
You'll still be able to view and export most data in iExplorer without this password since it's only the data in the Backup section that requires you to enter it (and only for encrypted backups). All other sections of iExplorer are a direct connection to your device and do not involve an iTunes backup at all.
iOS 10 iTunes Backups Only
First, for iOS 10 backups, you definitely need to use iExplorer 4 since iExplorer 3 doesn't support iOS 10. You can tell which version by looking at the app icon's color. iExplorer 4 has a white background, and iExplorer 3 has a blue background.
If you are absolutely certain that you are entering the correct iTunes backup encryption password for your iPhone or iPad, and it's definitely the same one you chose for encryption in iTunes and not a different and totally separate password such as the one you use for your AppleID or iTunes account, and iExplorer 4 is still saying decryption failed and invalid password, it may be due to a change Apple made in iOS 10.2, and you should just need to update iExplorer 4 to the newest version. To update iExplorer 4 in Windows, just simply close and reopen it while connected to the Internet. To update iExplorer 4 in on a Mac, click on iExplorer menu > Check for Updates. Once you are on the most current version of iExplorer 4, try accessing that same encrypted backup again in iExplorer and see if your password now works. If it still won't decrypt the iOS 10 backup, please go through the rest of the article above that talks about trying your other known passwords because you are likely just entering the wrong password.
Apple added an entirely new encryption method in iOS 10.2 that is even more secure which makes it nearly impossible to guess and hack someone's backup via "brute force" (guessing several passwords until one works). iTunes even encrypts the metadata for your files when backing up an iPhone or iPad with iOS 10.2 or later.