What does "public api" mean in Semantic Versioning?

I'm learning about how to assign and increment version numbers with the rule called "Semantic Versioning" from http://semver.org/.

Among all its rules, the first one said:

Software using Semantic Versioning MUST declare a public API. This API could be declared in the code itself or exist strictly in documentation. However it is done, it should be precise and comprehensive"

I am confused about "public API". What does it refer to?