What is exactly a file-like object in Python?

In Python, a file object is an object exposing an API having methods for performing operations typically done on files, such as read() or write().

In the question's example: simplejson.load(fp, ...), the object passed as fp is only required to have a read() method, callable in the same way as a read() on a file (i.e. accepting an optional parameter size and returning either a str or a bytes object).

This does not need to be a real file, though, as long as it has a read() method.

A file-like object is just a synonym for file-object. See Python Glossary.

From the glossary:

file-like object

A synonym for file object

and a file object is

file object

An object exposing a file-oriented API (with methods such as read() or write()) to an underlying resource. Depending on the way it was created, a file object can mediate access to a real on-disk file or to another type of storage or communication device (for example standard input/output, in-memory buffers, sockets, pipes, etc.). File objects are also called file-like objects or streams.

There are actually three categories of file objects: raw binary files, buffered binary files and text files. Their interfaces are defined in the io module. The canonical way to create a file object is by using the open() function.

File-like objects are mainly StringIO objects, connected sockets and, well, actual file objects.

If everything goes well, urllib.urlopen() returns a file-like object supporting the necessary methods.