there are several ways to put it. basically, these combos are the ones where you have two genes from the same gene complex in the same BP. some call it an "allelic" combo, some call it "acts like a super", or "combined super-form".

they are listed here: http://www.owalreptiles.com/complexes.php

common examples would be many BELs (if they are made from two different genes), and vanilla cream, and things like super stripe / puma / highway. basically its a bunch of genes that sit at exactly the same location in the genetic code. so its impossible to make a "super fire vanilla" or a "super vanilla fire" or a "super butter mojave". when a gene complex is involved, these genes interact in a very strong way, and you cannot get more than two of them in one BP.

its often also an advantage: when you breed vanilla cream to vanilla cream, all offspring will get two genes (no single-gene or triple gene offspring), a bit like breeding super pastel to super pastel. except that you get 25% super fire, 25% super vanilla, and 50% vanilla cream. or when you breed highway to highway (its yellowbelly + gravel), you get 50% highway, 25% ivory (super yellowbelly), 25% super gravel.

slowcountryballs is correct: you can breed a vanilla cream to any number of normals, you will not get any normal hatchlings. half of them get vanilla, half of them get fire, no exception. but you also wont get more vanilla creams.

people often intentionally focus on genes from one gene complex, as a project, and work with it. for example, if a breeder just loves mystic potion (/ purple passion), that person will have mojave combos in the collection, and combos with mystic(/phantom), maybe super mojaves, maybe super mystics (or super phantoms), and mystic potions. and its the same with vanilla cream.