Hummmm...it's kinda hard to explain in writing, Bita. Here's an analogy. If you had a direct telephone line to your spirit, or you could call it your "center", the wise, calm part of yourself, you could "talk" to it all the time. Unfortunately, the mind is "chattering" constantly (all those thoughts that go round and round) and it's like trying to talk on the phone in a room full of chattering people. Distracting!! The object of meditation is to teach the mind to "shut up" and listen by focusing on something like a mantra or your own breathing, in other words, by giving it a simple task to do to keep it quiet you can "concentrate" on listening to your inner self.
There are lots of books out on the subject, but I myself have a spiritual teacher - just like I need Cathe for the physical stuff (some people just strike out on their own - runners, for example, but many of us need the inspiration of a teacher or guru - like our Cathe!). And yes, by all means, split sessions are fine - even 15 minutes a day is wonderful! Like anything, consistancy is the key. The path of meditation is a life long journey, but well worth it.
Hope this answers some of your questions.
Robin