In short
Caloric restriction mimetics are compounds intended to reproduce some of the cellular signaling effects of caloric restriction — such as AMPK activation, mTOR inhibition, or autophagy induction — without requiring an actual sustained reduction in food intake.
The category was named by researchers looking for a way to capture the health benefits reproducibly seen with caloric restriction in animal studies, without asking people to sustain a difficult, long-term reduction in calorie intake. Candidate compounds are grouped here because of a proposed shared mechanism — nutrient-sensing pathway modulation — rather than a shared chemical structure, and include metformin, resveratrol, and spermidine, among others.
The strength of evidence varies enormously between individual compounds in this category, which is why it functions better as a mechanistic hypothesis for organizing research than as a single validated drug class. Metformin has the most substantial human safety and epidemiological data (from decades of diabetes use) but the dedicated longevity trial (TAME) had not reported definitive results as of this writing; resveratrol's proposed mechanism has been scientifically contested; spermidine has more limited but growing human trial data on some biomarkers.
None of the compounds discussed under this label has regulatory approval for an anti-aging indication in any major jurisdiction — each is evaluated, and should be understood, on its own individual evidence base rather than by association with the caloric-restriction-mimetic category as a whole.