In my most general understanding and in the context of this site, secular simply means "not AA".
Using a greatly over simplified interpretation, there are two general recovery approaches: 1) the AA/12 step approach that is based on admitting powerlessness over alcohol and on turning one's life and will over to the care of God; and 2) everything that is NOT the traditional AA/12 Step approach, which includes rational recovery, SMART, LifeRing, etc. The secular approaches are generally favored by those who want to be empowered, are atheist or agnostic, and/or prefer a rational/logical recovery method.
This gets confusing because it is applied along a range of adherence. There are atheists in AA and there are religious people who follow a secular recovery approach. There are secular interpretations of the twelve steps. There are people in AA who use the secular recovery tools and people following secular approaches who use AA tools. People put together an approach that works for them.