What is it when you see a "kept" receipt voice imessage after you send a voice memo to someone? Does it mean that they read it? Or that it was deleted?

When you send a audio message it'll say 'Keep' by default which means you can click it prevents the auto expiration, so going off of this if your seeing 'Kept' I assume it means the receiver has saved it. You can turn off the auto expiration if you wish. See Send photo, video, or audio messages on your iPhone, iPad, or iPod touch - Apple Support for more information.