One of the biggest changes to the program this year is the inclusion of nearly two extra months in the program timeline. We got some great feedback from our students and mentors last year that three months is often too little time to read necessary documentation, write code and participate in the organization's community in-depth. Since our not so secret desire is for our students to stick around and keep contributing to their organization's code base long after the program ends, we figured we'd give you some extra time to get up to speed on the community side.
In practice, the community bonding period is all about, well, community bonding. Rather than jumping straight into coding, you've got some time to learn about your organization's processes - release and otherwise - developer interactions, codes of conduct, etc. We also figured it would be easier to socially engage with your fellow developers when the pressure to ship isn't looming in your vision. I know few folks who didn't lurk in a project's IRC channel for weeks or even months before submitting their first patch, let alone saying hello and getting to know the other folks in the channel.
If you've already worked with with your organization's code base and developers, great! Take this time to help out your fellow students who haven't. There's a great deal of undocumented lore in every community, and your previous experience can be invaluable to them as they get started.
And why aren't you mentoring already? ;)