Everything exists in a socio-cultural environment, and while society relates more to a top-down organization of human life, you can say that culture relates to how human life develops organically in a bottom-up way.
Simply defined 'the ideas, customs, and social behaviour of a particular people or society', you can simplify it further to 'how people do things, and why'.
As a designer you can study culture very directly through observations, but also come to understand deeper layers of human behavior, what are the underlying human relationships, thought processes, beliefs and values of the people you design for. In modern Western societies, the term 'pressure cooker' signifies a new way of working with high efficiency and people from multiple cultures and disciplines working together. In some communist regimes, it is a cooking device people use because it is recommended by their leader because of its efficiency. It hangs together with a common way of doing things - culture.