Please, I’m very confused. Context is medical.
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When looking at cells in a microscope, you usually need to use some sort of a colored stain, because cells are mostly transparent and hard to tell apart. One common stain is a *Gram stain* (no relation to the unit of mass, the “Gram” here is a guy’s name), which is a dark purple dye washed out with a light pink dye.
A bacteria whose cell wall holds on to the Gram stain shows up as purple afterward, and we call these bacteria *Gram positive*. A bacteria whose cell wall doesn’t hold onto it is stained a light pink by the second bit of the Gram stain, and we call these bacteria *Gram negative*.
It turns out that this corresponds to a piece of the bacteria’s anatomy: Gram positive bacteria have a thick layer of [peptidoglycan](https://en.wikipedia.org/wiki/Peptidoglycan) in their cell walls, while Gram negative bacteria have a thin layer. It’s also medically relevant, because in general Gram positive bacteria are more vulnerable to antibiotics.
More broadly, Gram positive/negativity is a very common and very easy method of narrowing down what kind of bacteria you’re looking at. For example, *Y. pestis* (plague) is a Gram-negative species, so if you’re looking at a bacteria and it takes the Gram stain, you know it’s *not* plague.
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