Chippy, it's a really easy idea.
In a nutshell, it's the idea that whatever key you are playing in, the 1st- the 4th and the 5th step in the scale is the basis of blues music/rock/etc.
AS in..
If you are in "E", you count up e=1, f=2, g=3, a=4, b=5, c=6, d=7, and the octave above of 8=e.
Depending on what you are playing, it can be as easy as identifying the first note you play.
If it's an "F", F=1, g=2, a=3, b=4, c=5, d=6, e=7 and the octave above f=8.
8 steps in a scale, sometimes less. Pentatonic Scales are scales of 5 tones. (penta= 5)
All the old blues is in 1-4-5, even if you do not realize it.
You take a 12 bar blues pattern and adhere the 1-4-5 to it.
http://en.wikipedia.org/wiki/Twelve-bar_blues
1-4-5 also equals I-IV-V, they are just using a roman numeral to designate this.
IF YOU ARE A SINGER and you know you are singing in "A" or your first note is "A", you could then ascertain that the backbone (bass part?) of a blues type sound could be 1-4-5 or A=1, D=4 and E=5.
I don't think anyone plays it like this, without variation. The variation is understood else it would be oversimplified when you play the piece.
Here's another article that further defines it.
http://wiki.answers.com/Q/What_is_a_1-4-5_Progression
So you take your 1-4-5 method,
Pentonic Minor/Major scales minimum,
The open B7 style chord, with reference to the e,d and a positions
and you basically have the keys to the basic blues.
The 1-4-5 that you hear is in the background and is like a rhythm guy asking a lead where he/she is going next and having them actually call out a chord. It gives you a very good idea of where everything is going next, regardless of whether or not you actually know.
When you play with other people you never played with before, it really is an advantage. When you see people playing well within a group and they suddenly lose it during during the midst, chances are someone doesn't know this little tidbit.
I know a bassist that performs ok, but when faced with a new guitarist he is very adept because even before the guitarist goes off on some f**k up tangent that leaves everyone else lost, this guy is figuring where the tune is going to end up as in WHAT NOTE WILL BE PREVALENT at the end.
For this reason, everyone wants to play with him. He is a master of improvisation and in reality, he is what I call "no frills" or "the MSB brand of bass players" as he isn't terribly talented, but his theory is SPOT ON!~