iExplorer now lets you access any encrypted iPhone or iPad backups in version 3.3 or later where you just need to enter your encryption password for that device. The encryption password is one you would have chosen when setting up your iPhone, iPad, or iPod Touch 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 not at all related to iExplorer and is one you would have chosen before ever using iExplorer. This Apple article explains more about it:
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.