"The Heartbleed bug allows anyone on the Internet to read the memory of the systems protected by the vulnerable versions of the OpenSSL software. This compromises the secret keys used to identify the service providers and to encrypt the traffic, the names and passwords of the users and the actual content. This allows attackers to eavesdrop communications, steal data directly from the services and users and to impersonate services and users."
Here's Apple statement:
“Apple takes security very seriously. IOS and OS X never incorporated the vulnerable software and key Web-based services were not affected,” an Apple spokesperson told Re/code.
You can learn more about Heartbleed here.