Chickens have many genes. A gene is the protein string stored at a specific place on a chromosome. For each of these genes there are different alleles (variations) that can fill that place. We typically call that location the "such-and-so" gene, using the term for a specific one of the alleles.
Think of it as being a child's shape sorter puzzle: Each hole is a different shape, and only blocks that are the correct shape can fit into that opening. However, any number of different blocks of the correct shape, but that differ in colour or pattern or material or whatever else; but htey are all the SAME shape. Let's further say that each opening has room for two blocks, and the rule is that you have to fill each opening--you can't leave any empty space.
Let's label one of the openings "andalusian blue." There are only two different types of blocks for this space: one we'll call blue and the other "not-blue." So, for this particular opening you have three choices: two blue blocks, two not-blue blocks or one of each.
A different opening also only has two different types of blocks. This is the lavender opening, and the choices are two blocks of lavender, two blocks of not-lavender or one of each.
A third opening has a bunch of different block types: extended black, birchen, wheaten, brown & not-black. There are a while lot more choices here as you can have two blocks of any of these or two different blocks--but they could be black & birchen or black & brown or black & wheaten or black & not-black or birchen & brown or birchen & wheaten or birchen & not-black or wheaten & brown or wheaten & not-black or brown & not-black (I think I got them all, but if not, at least you should get the idea.
And then there is a 4th, and a 5th and a 6th and ..... opening.