Toxicity is the property of being poisonous. Every chemical capable of dissolving in water is toxic to some degree but some are much more toxic than others. There are different measures of toxicity and they mostly try to measure how much of a substance is required to kill a subject.
Selective toxicity is a term we use to target and kill certain cells in the body without affecting other organs as far as possible.
In cancer treatment, you will frequently encounter that where you target only the rapidly dividing cancerous cells.
And of course, unfortunately, other normal rapidly dividing cells also get killed. Like the Hair follicles, leukocytes (WBC) Etc.
Selective toxicity is one of the essential parameters kept in mind while designing a drug molecule. More selective toxicity means a better drug.
For example, penicillin, the first Antibiotic, shows selective toxicity towards bacterial cells and not human cells because penicillin targets the cell wall of the bacteria and kills it. But since human cells don’t have cell walls, they are saved.