Hucow is used to describe a fairly wide range of things but basically the idea is a woman who identifies as a human cow.
At a minimum this will usually include large breasts, referred to as udders, with a focus on lactation. In addition they often take on other cow like traits such as a high degree of objectification as a service animal.
In extremes cow masks are worn, milking machines employed (yes this really happens) and there are even herds that group together.