Well, if you don't understand it, you must read some basic book about modern computer architecture.
Short answer: It is possible to reserve memory address space without assigning actual physical memory. As your program runs it can dynamically assign physical memory pages to its address space when it needs it.
Again - you must read a book if you want to understand it.
Why downvoting?
It may be not the best explanation, but IMO the closest to the way a "developer" should understand it.
On Linux a virtual memory is committed with mmap call.
Check this for details:
http://stackoverflow.com/questions/2782628/any-way-to-reserv...
This is basically how shared libraries and other shared objects are attached to your process.
It can be when a short description completely and accurately describes what's going on, but the person creating the comment needed to read a few books on the subject to actually understand both what's happening and why due to other issues.
EX: How does virtual memory get mapped to L1 cache?
to henrikschroder:
I expected that this reaction was caused by mentioning a book.
It is my deepest belief that questions like memory management can not be answered with brief comment.
You must read at least a basic book if you want to understand it in depth.
