Epsom salt is hydrated magnesium sulfate.
It is a bitter, colorless or white crystalline substance. It is used as a laxative and as an anti-inflammatory soak. It may be called for in some old time jelly recipes because the magnesium ions are capable of forming weak links with pectin thus helping make the jelly gel. It is not a recommended practice. If a gelling aid is need use one of the commercial pectin products.