endl and flushing the buffer

In the C++ primer book, in chapter (1), it mentions the following:

endl is a special value, called a manipulator, that when written to an output stream has the effect of writing a newline to the output and flushing the buffer associated with that device. By flushing the buffer, we ensure that the user will see the output written to the stream immediately.

What is meant by "flushing the buffer" here?