A gene is a section of genetic code that gives instructions for a specific purpose. This gene section can be inherited, and changed (mutation), which is how different species evolve a variety of appearances and abilities.
There is a difference between "gene" (a whole section of code related to hair in general) and "allele" (a unique version of that same gene section, like dark or light hair). So everybody gets a hair gene, and we each get different alleles (versions) of that gene.
Some genes are tiny, just a small chain of bases. Other genes are quite big and could have thousands of bases. So when you think of a gene, it is like a building block of genetic material. And just like with legos there can be little ones, big ones, and they all have different purposes - most of which we have not even discovered yet!