blinky
I cant go through it too thoroughly because each part has its own significance. I have taken bits and pieces from Artscrolls Interlinear Siddur. They have an explanation before every Tefillah in the siddur what its about and they also explain various phrases. Im a huge fan btw and I think its mandatory that any Jew who wants to really Daven with Kavannah should Daven from one of theses. I cant tell you how thankful I am to Artscroll for putting out this Siddur.
Ok here goes:
Shema before going to sleep is a protection against the dangers of the night. (Brachos 5a) Many of the Tefillos we say date back to the times of the Gemara and Geonic times.
The Ribono Shel Olam tefillah (the first thing you say) is because before going to sleep it is proper for one to examine his/her deeds of that day and should one recall an improper deed one should pray for forgiveness and undertake to correct their ways.
It is also proper to forgive those who wronged him/her. In the merit of this one will be granted long life (Mishah Berurah 239 1:9)
Hamapil is about that the ideas and fantasies that we have during our wakeful hours not produce disturbing nightmares or immoral dreams. Such dreams menace the purity of our thoughts and feelings even during our waking hours.
Blinky, there are a lot of other things we say in Krias Shema Sheal Hamitah and they are all important and many have deep Kabbalistic import.
If you want to see more go to your local sefarim/judaica store and look at an Artscroll Interlinear Translated Siddur at the KSS”H part.
Again I strongly reccomend you get an Artscroll Interlinear Siddur. It will change your level of Tefillah tremendously. I am willing to buy you one as well Bli Neder if you promise to use it daily and Daven with Kavannah.