This means that there is not a single implementation that defines the language, but rather an agreed upon document that any conforming C compiler must implement to be a C compiler.
Most mature C compilers will specify what version(s) of the C standard they implement so developer users can know what features are available. Many C compilers also implement non-standard extensions to the C language and libraries to be more competitive, overcome language shortcomings, or provide for specialized needs or development targets (e.g. features for embedded targets).