If the spike is on the very extreme right 255 reading it indicates chanell clipping. You have overexposed and one or more of the red;green;blue chanells is blown. Whether this is relevant depends on the scene...can you post a picture?
If it a small section of white or light reflection it won't really matter.
There is a lot to learn with digital, the histogram being one small part. The "white histogram you see on the K10 is a "luminance" histogram. This takes the combined readings from the seperate 3 colour histograms but "weights" it according to how sensitive the human eye is to that colour. You can also view the red;green ; and blue histograms on your K10 by using the up/down rocker button while looking at the luminance histogram.
A RGB histogram is a simple average of the seperate colour ones and is less accurate than the luminance one for general over/under exposure, but can disguise the fact that one single colour has "blown". This will generally occur when photographing strong solid colours (flowers for example).
As a rule if photographing "general" scenes without very bold colours use the K10 luminance histogram to check for overexposure. If photographing strong colours, pay more attention to the seperate coloured histograms.
Have a look here for some excellent tutorials
Digital Photography Tutorials