In vitro and in vivo
In vitro means 'in glass': experiments on cells or molecules in a dish. In vivo means 'in the living': experiments in animals or people. A drug that kills cancer cells in vitro has cleared only the first and easiest hurdle.
In vitro work (cell lines, purified proteins, organoids) is fast, cheap and controlled and is where targets are discovered and drugs first tested, but it lacks the immune system, blood supply, metabolism and toxicity of a whole organism. In vivo work in mice, often carrying transplanted human tumours, adds those features imperfectly; ex vivo means tissue taken from a patient and tested outside the body, and in silico means computer modelling. The gap between these models and patients is one of the main reasons most drugs that look promising in the lab fail in trials.