Oh it wasn't left unexplained. Oro said it out loud and clear. If you wait just 3 seconds longer and actually bother to watch the whole episode, you will hear Oro say: "What's the meaning of this, Tsunade?".
Course it was left unexplained.. it won't be explained til next episode, that's why they left us in the dark like that. Besides, just cause Orochimaru thinks Tsunade is the one deceiving him, does not mean he's right. He could be easily misunderstood, that's how betrayal works, playing off the mis-assumptions of the person you're betraying.

I've said it before, and it bears repeating: Just cause a character thinks he's right or says something, does not mean they ARE right.

kthnx
What is this? You've said it twice now and can't think of a way to pronounce it.