Skunk is a specific type of cannabis and is so-called because it has a very strong smell, but these days it has become the generic term for stronger forms of the drug. It is regarded as stronger because it contains much higher levels of the active ingredient in cannabis, tetrahydrocannabinol (THC). It also contains much lower levels of cannabidiol (CBD), which is an anti-psychotic substance that can moderate the effect of THC on the mental health of users.
The "skunk" cannabis is used to treat mental health problems.