The endoplasmic reticulum is made out of a lipid membrane, just like the cell membrane around the outside of a cell, or like vacuoles, or like the membrane around the nucleus of a cell. Probably it evolved about two billion years ago out of the cell membrane around the nucleus of the cell, just like vacuoles evolved from the cell membrane around the outside of the cell. All eukaryote cells have some endoplasmic reticulum, or the remnants of one they used to have. The word "endoplasmic" just means that it is floating in the cytoplasm of the cell. "Reticulum" is the Latin word for a little net. So the endoplasmic reticulum is a little net that floats in a cell's cytoplasm.
The endoplasmic reticulum, in fact, is still connected to the nuclear membrane that wraps around a cell's DNA. So there is a direct connection between the cell's nucleus and the endoplasmic reticulum. The lipid of the endoplasmic reticulum makes a maze of little tubes or tunnels.
Inside the tubes of the endoplasmic reticulum, RNA molecules drift out of the cell's nucleus and match up with protein and sugar molecules floating around in the tubes to make new proteins and amino acids and long chains of sugars and other kinds of large hydrocarbon molecules. One-celled creatures like amoebae use these molecules to repair their cell membranes and to break up their food into smaller, more usable pieces, and to reproduce themselves. Sometimes they send these molecules out of the cell to digest their food outside the cell before bringing it in.
Specialized cells inside multi-celled creatures like plants and animals also assemble more specialized molecules, that they can send out of the cell to do things in other parts of the body. For instance, animal cells produce insulin inside the endoplasmic reticulum. Plant cells produce long sugar molecules that they use to stiffen their cell walls so they can stand upright.
After the RNA is done assembling the new molecules, the molecules float away down the tubes to the Golgi bodies.