What was done
Statistical properties of climatically significant volcanic eruptions over the last 600 years were analyzed in an effort to estimate the probability of future eruptions over the next decade.

What was learned
The authors determined that there is a 35% to 40% probability of a volcanic eruption with the capability of producing a radiative perturbation of -1 Wm^-2 or larger in the next 10 years.  Such an eruption is estimated to produce a 0.1°C to 0.15°C cooling over a 2-3 year period.  The probability of a larger, Mt. Pinatubo-scale, eruption with a radiative perturbation of -3 Wm^-2 is 15% to 25%.

What it means
The authors are concerned with the idea that were a major volcanic eruption to occur in the next decade -- which seems a fairly good possibility -- it could "mask the CO2 effect and complicate discussions on a greenhouse gas protocol."  They are correct; but that's how the real world operates.