The thing is, how do you define a speed?
You do it by measuring out a distance. Then you need a clock, in your absolute vicinity. Then you need to decide what is 'still', which is easiest to do relative something big that you might be standing on, as our Earth :)
Then you 'clock' something that moves the distance you measured up, and so becomes able to define a speed to it, relative yourself on your uniformly moving, and therefore in most respects able to be defined as 'standing still', Earth.
So now you have a speed.
But the weirdest thing is that you can have different uniform motions on that thing you define as being 'still'. There is nothing stating that all uniform motions in the universe must be the same, as measured from Earth. There are all sorts of uniform motion in the universe, some faster that Earths, others slower. And the idea behind 'c' being a constant is that no matter what uniformly moving 'frame of reference' (as our Earth) you stand on, that speed must come out the exact same, when you measure lights speed in a vacuum locally.
So what is the speed of light?
Well, it's a constant firstly, a 'speed' secondary as I see it.
That one is the most important thing to realize about radiation. That is is a constant, 'speeds' doesn't make sense for it. Except as defined locally from a uniform motion, in which case it has the same 'speed' everywhere. Eh, and yes, that one has been tested to smithereens, over a hundred years or more.. And I agree, it's weird..
All speeds we have are arbitrarily defined relative what we find to be 'still'. What Einstein did was to define what type of frames of reference you could call 'still', and those are all uniformly moving. So the whole question of 'speeds' becomes slightly twisted as we don't have a 'absolute frame of reference' that we can measure all other 'speeds' from.
But if you accept that all uniform motion express itself the same, then you have one sort of 'frame' you can use, but then a 'speed' also becomes relative you measuring. And that makes sense, because when you measure you do it relative your wristwatch and your ruler. You don't call Andromeda and ask them, if you did, it would be their measurement, not yours.
And one more thing that may not be so obvious at first.
If light always must be the 'fastest' thing you can measure, giving you a same 'constant speed', no matter what 'speed' you find your uniformly moving (earth) you stand on to have, relative some other object.
Do you think that anything other, as some rocket, that you measure can outdo that lights 'speed'?
And if you find it impossible, do you think your friend being at another (uniformly moving) 'frame of reference', having a different 'speed' relative your earth, can find the rocket to move FTL?
Just imagine yourself teleported to him and wonder what you will see.
You might also ask yourself, if now light/radiation always have one 'speed'? And if all I measure is a result from interactions, relative the 'information carriers' that light/radiation becomes for my measurements, and senses. What would happen if something really could go FTL?
Would I even see it?
cleaned up the spelling etc, a little :)