Well first, let us ignore Arius and his squabblings that have been preserved by various groups to date. Jesus is God, plain and simple, in the orthodox style. (That is not to say that a Mormon poster or anyone else can’t provide their ideas, but for all my purposes here.)
From my understanding, the Son is eternally being begotten: He is generated by the nature of the Father for all eternity. This simply cannot change because God’s nature does not change. He is eternal, and the Father cannot stop producing Him (which would be as if God just ceased to exist: He can’t!)
Interestingly enough, three sources I found for the eternal generation of the Son are all Calvinist. (Protestant or Calvinist sources do not have a differing version of theology proper, so it’s fine for us to use.) The teaching is summed up as this:
The eternal generation of the Son… is the teaching that the Son is eternally begotten by the necessary will of the Father, but that the Son is not created or caused, and that neither the Son nor the Holy Spirit are dependent upon the Father or any other member of the Godhead for existence.
The eternal generation of the Son is a statement on the relationship within the Trinity between the Father and the Son before the incarnation. Therefore, the term is not in reference to causation but to nature and relationship. The eternal generation of the Son must be understood to mean that the Father did not bring the Son into existence, which would deny the full immutability and deity of the Son. [Source]
This article is the best I have seen yet on the matter, with a simpler definition than the above: theopedia.com/eternal-generation-of-the-son