After August 1st there are likely to be two versions of the Bitcoin blockchain and two separate digital currencies - Bitcoin (BTC) and Bitcoin Cash (BCC).
"In the event of two separate blockchains after August 1, 2017 we will only support one version. We have no plans to support the Bitcoin Cash fork. We have made this decision because it is hard to predict how long the alternative version of bitcoin will survive and if Bitcoin Cash will have future market value." - stated Coinbase Team in their blog post.
If you want to have access to both BTC and BCC you should withdraw your bitcoins from Coinbase before July 31.
You can find more information about bitcoin fork here.