It's probably traces of certain elements that mean something about the possibility for past life on Mars (but not proof of life)...which won't be too exciting for the general public.
Which has the best resolution of all the Rover's tools? Obviously we didn't send a tunneling electron microscope to Mars, so what scale of objects can we detect? Bacteria-sized objects?
It's not like that. The rover has several imagers including a microscopic(ish) imager, but it can't see things as small as cells. Almost all of the instruments are designed around determining the mineral composition of rocks or the chemical composition of samples.
If there are signs of life on Mars it's unlikely that we'll be able to see it with any imager on the rover, but we might be able to detect its chemical signature. The sample analysis instrument is a gas chromatograph connected to a laser spectrometer and a mass spectrometer. What happens is that a sample will be vaporized and then pushed through a long column. The column is designed so that different molecules will have a different speed through the column and thus will come out at different times. Then each of those parts of the sample (which should be comprised primarily of one molecule) is passed into the spectrometers, which can identify its molecular structure. This can determine the chemical makeup of minerals, but it can also identify complex organic molecules such as the components of cells or perhaps things like cellulose or lignin and so forth.
One of SAM's gas chromatograph columns is chiral: it can separate left-handed molecules from right-handed ones. Terrestrial life uses amino acids that almost all one handedness, while Earthly sugars are mostly the other. Finding a similar pattern on Mars would be a smoking gun demonstration of extraterrestrial life.
Exactly this. We'd expect for random abiotic organic chemistry to yield racemic mixtures of chiralities. Detection of one enantimer at higher levels than another typically calls for enzymes.
