Methane on Mars - what does it mean?

The short answer: Mars is either geologically active or biologically active. The latter would be more startling, of course, but the planetary scientists, the first would matter a lot, too.
Scientists inside and outside NASA have spent five years examining the sometimes-fragmentary evidence from spacecraft and telescopes and building the case for methane emissions from the surface of Mars.
As this article in Science puts it, "...Methane-generating bacteria might be living off hydrogen produced by rock...Or, purely inorganic reactions between water and rock rich in the mineral olivine could do it."