what does it tell me? If the static thrust of a 12x6 prop @ 12K RPM is 3.63kg? what do I need to know about this?

Static thrust means how much thrust is resulted from the prop spinning stationary. You can test this by mounting your engine in a stationary location, running the engine and taking a thrust reading on how much the engine is pulling. It will be different than dynamic thrust (which is the thrust that is seen in the air). If I remember right, static thrust values are always greater than dynamic thrust values.

But really, it doesn't really tell you much of anything. You can use that information along with the airframe drag characteristics, weight, rolling friction to estimate acceleration. And if you know liftoff speed, then you can estimate the takeoff roll, but you really don't need to know these things for a model airplane, unless you are in some sort of competition or something (which is why I know about it ).

It will give you an idea of your vertical performance. If your thrust:weight ratio is greater than 1, you start to get into the "unlimited vertical" performance range.

That this combination is suitable for a plane in the .40 to .60 size range depending upon what performance you expect.

An 8lb .60 plane will not have a better than 1:1 ratio with that.

However a 5-6lb .50 size plane will reach for the stars.

Need to know that? or anything about it?

Yeah, it's basically useless by itself.
What kind of plane you'll be using the prop on matters. The engine matters. Your flying demands matter.

Matter of fact, your engine might not be able to turn 12K with that prop, or would have to be throttled back to 12K.

But it might give some comparative value if you are able to compare to other props on that engine. That is, if you can read thrust numbers, and can do them on the same day (or know how to adjust for density altitude etc)

Nah, not much value in those numbers by themselves. Want to know if that prop works better than others? Try 'em at the field. You'll know in just a couple of flights.

I would assume that you could compare 2 engines if you measure the rpm and prop and static thrust to find out if one truly does have more power then the other one?

I think I got my answer from opjose... thanks to all you guys also...