Hi XP,
it’s all covered in help and the forum. 
It means that RC thinks the error is too big.
I think above 2 the number gets red and the triangle appears.
It is the difference between your definition of a singular point (via CP) and RC’s tie points in the area in pixels.
Activate a 2D view and go to scene and tick “show residuals” or something like that.
Then you can see orange lines which will give you a better idea of alignment problems.
In your case, that’s probably a good sign because RC has had some trouble finding the right matches.
Just define a few CPs and align again.
If all the errors are below 0.5, it should be fine.
However, it is likely that there will now be other spots where the misalignment will have increased.
Just continue this procedure until you are satisfied.
And yes, it does take time and effort, there is no way around it apart from taking more images (which can be much quicker).