Simply stated (kinda
Simple carbs are the processed white group...white bread, 'standard processed pasta'
In that regard, the simple carbs above are the regular carbs
Complex carbs would, among other things, be the whole wheat version.
Sweet potato versus baked potato
whole wheat bread vs white bread
Whole wheat spaghetti/angel hair etc vs standard processed pasta
Essentially, the processed standard foods have most of the nutrients and fiber processed out of them. It's the fiber taken out that plays a major function in slowing down the glucose (and then insulin) spikes that lead to obesity and eventual type II
That's where adding a complex carb of another kind, helps to offset just the steak and baked or mashed potato meal.
A even much more general description off of Wiki touches briefly on this as well