Programs are often organized in a modular fashion. Lower-level operations are grouped together, and called by higher-level functions, which may in turn be used as basic operations by still further upper levels.
For example, the lowest level might define a very low-level set of functions for accessing a hash table. The next level might use hash tables to store the headers of a mail message, mapping a header name like "Date" to a value such as "Tue, 13 May 1997 20:00:54 -0400". A yet higher level may operate on message objects, without knowing or caring that message headers are stored in a hash table, and so forth.
Often, the lowest levels do very simple things; they implement a data structure such as a binary tree or hash table, or they perform some simple computation, such as converting a date string to a number. The higher levels then contain logic connecting these primitive operations. Using the approach, the primitives can be seen as basic building blocks which are then glued together to produce the complete product.
Why is this design approach relevant to Python? Because Python is well suited to functioning as such a glue language. A common approach is to write a Python module that implements the lower level operations; for the sake of speed, the implementation might be in C, Java, or even Fortran. Once the primitives are available to Python programs, the logic underlying higher level operations is written in the form of Python code. The high-level logic is then more understandable, and easier to modify.
John Ousterhout wrote a paper that explains this idea at greater length, entitled ``Scripting: Higher Level Programming for the 21st Century''. I recommend that you read this paper; see the references for the URL. Ousterhout is the inventor of the Tcl language, and therefore argues that Tcl should be used for this purpose; he only briefly refers to other languages such as Python, Perl, and Lisp/Scheme, but in reality, Ousterhout's argument applies to scripting languages in general, since you could equally write extensions for any of the langauges mentioned above.