Elder is Sambucus nigra, and it is a case where the folk enthusiasm and the actual chemistry both need stating.
The flowers and the cooked ripe berries are food. Elderflower cordial, fritters and wine are ordinary preparations; elderberry is made into wine, syrup and preserves. There is a real and continuing body of clinical investigation into elderberry extracts and the duration of influenza and cold symptoms, with some positive trials, small sample sizes and unresolved questions about preparation and mechanism. It is one of the more plausible folk remedies and it is not established.
The rest of the plant is another matter. Elder leaves, bark, roots, and unripe or raw berries contain cyanogenic glycosides, principally sambunigrin, which release hydrogen cyanide on digestion. Raw or under-ripe berries in quantity cause nausea, vomiting and diarrhoea, and there is a documented mass poisoning incident in California in 1983 from juice pressed from berries with leaves and stems included. Cooking destroys the glycosides, which is why every traditional preparation of the berry involves heat, and why a tradition can be practically safe without anyone in it knowing why.
Culpeper describes elder as a whole medicine chest and uses every part, which is a fair description of how it was treated. He also assigns it to Venus. That attribution is worth flagging, because it sits awkwardly with the folklore.
The tree is structurally odd in a way that shaped its uses. Elder wood has a wide core of soft pith which pushes out easily, leaving a natural tube, so elder was the standard material for whistles, pipes and bellows nozzles, and the name Sambucus is generally connected to a musical instrument.