According to the Wall Street Journal, Reuters, and the Associated Press, Google had to agree to keep Android free and available for anyone to use for at least the next five years in order to gain China's approval to purchase Motorola Mobility. There's no more information given on the exact reason for this request, but it seems likely that it was included to allay fears that Google would give Motorola preferential treatment compared to other Android manufacturers. This stipulation removes the possibility of Google closing off Android to other OEMs — though it's worth noting that Google has never given any indication that it was considering such a move.
Of course, Android isn't technically the property of Google — it was created and developed by the Open Handset Alliance, of which Google is a member. While it's obviously the biggest contributor, Google probably couldn't just close off Android from other manufacturers without a bit of a fight. It's also worth noting that Chinese carriers and manufacturers often put forked and heavily customized versions of Android on their devices — this deal keeps Google building and developing the basis for their customizations.
While the deal has yet to close, it appears that this stipulation won't slow things down at all. A Google spokesperson said that "our stance since we agreed to acquire Motorola has not changed, and we look forward to closing the deal," while a Motorola spokesperson said that "we are pleased that the deal has received approval in all jurisdictions and we expect to close early next week." Google's strategy of keeping Android free and open to all has served it well over the last few years, so it doesn't surprise us to hear it plans to continue on that path.