If the result of a measurement is i.e. $3.2 \pm 0.7$, what is 0.7? At which confidence level we know that the real result is inside of this interval?
As your link indicates, it is the uncertainty in the measurement. The exact meaning of this can depend on context, but most of the time it is safe to assume that this is the standard deviation in the value that you should expect if the measurement were repeated a large number of times.