For security purposes, your password is never sent to Open Whisper Systems. It is known to your Android phone only, and we cannot reset it. Your password is used to encrypt your local message database.
Signal Android was deliberately designed to never reveal your password to anyone and to leave you entirely in control. If you are approaching customs in a foreign country, or you get pulled over by a police officer, or you are told by a killjoy teacher that you need to hand over your phone for texting in class, you can quickly lock Signal and feel confident that your messages cannot be read. This is because they are protected by a strong passphrase that only you know and that only you can change.
If you truly have forgotten your password then your messages cannot be recovered. If it were possible for you (or us!) to do this it would also be possible for the customs officer, police officer, or teacher in those hypothetical examples to simply pretend to be you and do the same thing (or ask us to do it). That's not acceptable. Password recovery options weaken security, and we want Signal to be very strong.
You can uninstall and reinstall Signal Android, which will give you the ability to start over and choose a new passphrase. You also will have the option to enable passphrase protection if you'd like, thereby turning on local encryption. But hopefully your memory was jogged while you were reading this explanation and you aren't even going to finish this sentence because you are already typing your just-remembered password into your phone.
Learn how to get started on Signal Android.