Here's an excerpt of the source code of the FileInputStream class from OpenJDK 8 (link):
/**
* Opens the specified file for reading.
* @param name the name of the file
*/
private native void open0(String name) throws FileNotFoundException;
// ...
private native int read0() throws IOException;
/**
* Reads a subarray as a sequence of bytes.
* @param b the data to be written
* @param off the start offset in the data
* @param len the number of bytes that are written
* @exception IOException If an I/O error has occurred.
*/
private native int readBytes(byte b[], int off, int len) throws IOException;
The native keyword (see this other Q&A) means that these methods are not implemented in the Java source code for this class; they are provided by the implementation of the Java interpreter which executes the program (e.g. the java command-line utility, which executes Java bytecode). The Java interpreter itself is ultimately written in a language like C, which has the low-level features required to actually implement these native methods. When the interpreter has to execute a method marked native, it invokes the corresponding function written in C.
Similarly, other high-level languages like Python have some functions which are implemented "natively" in the same sense. These functions, along with the behaviours of basic arithmetic and comparison operations, are "intrinsic" as compared to the large bulk of a language's standard library, which is usually written in the language itself.
