First, congratulations!
The condition could be any of a number of things (and, by thunder, there are some weird conditions: have one myself).
The only thing I can think of is that you say "master's grades": have you graduated from that degree, yet? If not, the offer might be conditional on completion of that degree.
Then again, it could (in theory) be something as simple as checking your references or seeing sight of your official transcript from other degrees.
I imagine there will be more experienced, better qualified individuals able to post on this. I just know how frustrating it can be when you're waiting to understand conditions.
What will likely happen is that, if the offer is made, it will clearly state what the conditions of that offer are. I would, if I were you, leave it a couple of weeks and then - if you haven't heard back at that point - send them a little email just asking for an update.
Hope it works out :)