"unusually high" in this case was a few tens of ppb (low concentrations in the parts per billion range) according to the article I read a few minutes ago. For comparison, methane is at about 1 ppm (1000 ppb) on earth. Normally on Mars, methane is down at the few ppb concentration, so these spikes aren't even a tenfold or hundredfold jump. Just a quick period where methane is at levels a few times above normal, so not a big deal in terms of main processes, but unusual just in terms of how methane behaves there.
What does it mean? Well, it likely means that there has been a release of methane that was fixed into the soil in some way (some sort of solid probably) and conditions (likely temperature) caused some of that methane to become gas and leave the soil. The spikes appear to have a seasonal timing which is why temperature is a probable trigger of a release. One important thing is that the measurements have come from a rover, down on the surface of the planet, but the satellite instruments we have around Mars have not yet identified any such anomalies.
Methane is fairly common when there is a lot of life (as we know it, carbon-based life made from organic molecules). However, methane is also a not-rare product of inorganic reactions on earth and elsewhere, and given the low concentrations and rareness of these "spikes", an inorganic origin is most likely the cause.
We can't get enough of the proper measurements to say a lot about how the methane comes into existence. Parts per billion isn't a heck of a lot of methane, and the anomalies when they occur are fairly localized and of short time duration.
Basically, what the methane comes from isn't known yet. It is rare enough that it is news even when these small whiffs of methane get detected. If it is from life activity, the obvious conclusion would have to be that there is very little active life on that planet, because we almost never detect methane, and when we do, there isn't a lot.