For years Google has maintained their own set of dozens of patches atop OpenSSL that haven't been upstreamed over breaking API/ABI compatibility, being too experimental, etc. In light of all the OpenSSL fallout this year and wanting to take a different direction in handling their mass amount of out-of-tree patches, Google is no longer basing their work atop OpenSSL upstream but rather will be importing new OpenSSL changes into their code-base.
Google doesn't intend to replace OpenSSL as a project but they will be pulling their new "BoringSSL" work into Chromium and Android. Google will also continue to send bug reports to upstream OpenSSL, continue their existing sponsorships, etc. BoringSSL will also pull in changes from OpenBSD's LibreSSL.
More details on this "BoringSSL" work by Google can be found on Adam Langley's blog.