The last thing a fish would ever notice would be water what does it mean?

It means that we are often unaware of the context in which we live... until it changes. For example, because the culture (context) in which one grows up is typically taken for granted, we may never consciously think about it. Often awareness only arises when faced with a new or changing context. The saying is also used in other settings, such as business. For example, a company may take its industry, products or technologies for granted until any or all of them are changed by another company, a technological evolution, etc.