There is a clause in the work contract that claims that Google has ownership of all your IP relating to Google's current or planned businesses. Given that it's Google, that's... a very broad category.
You can apply (the process is described on the same website as TFA) to have Google declare that a certain project definitely is not one of their businesses and therefore not covered. You could also make your own judgment, but (having talked to a lawyer about this when considering a Google offer) my impression is that that is a very bad idea and you should go through the process.
(I only know about the US-but-not-California version of the contract; it might be different in other jurisdictions.)
You can apply (the process is described on the same website as TFA) to have Google declare that a certain project definitely is not one of their businesses and therefore not covered. You could also make your own judgment, but (having talked to a lawyer about this when considering a Google offer) my impression is that that is a very bad idea and you should go through the process.
(I only know about the US-but-not-California version of the contract; it might be different in other jurisdictions.)