This was a light made from the purest olive oil that was to burn in the Tent of Meeting and later in the Temple. It is not clear whether this light actually burned continuously, or was rekindled every evening, but from the text it appears that it was relit daily. Today, every synagogue has a ner tamid at the top of the ark that holds the Torahs.
Most modern synagogues use an electric light.
In most commentaries the ner tamid is seen in allegorical terms. Unfortunately, when we switch from olive oil to electricity, most of the original symbolism becomes difficult to retain. It comes from olive oil which comes from the olive tree. This olive tree is none other than the Etz Ha-Da'at , the tree of knowledge in the Garden of Eden. In religious symbolism, every tree, from Buddha's Bo tree, to Jonah's gourd tree, to Christ's cross, can be the Etz Ha-Da'at, which may be more appropriately called the tree of wisdom.