One of your other questions today was about ethnocentrism. In a sense, cultural relativism can be seen as the opposite of ethnocentrism.
When we are ethnocentric, we judge other people based on our own cultural values. If, for example, people from another culture have different attitudes towards women, we tend to see them as backwards compared to us. When others do not conform to our expectations as to what is right or good, we judge them in a negative way.
Cultural relativism is the belief that other cultures should be judged by their own standards. Instead of thinking that our way is best, we should attempt to see other cultures from their own point of view. We should try to understand them through their own eyes. For example, in the instance of a society that allows polygamy, we should try to understand how the circumstances of that society could make it logical to follow that practice. Instead of condemning it because it is not in accordance with our own views, we should try to understand why it makes sense for that culture.
Thus, cultural relativism is the idea that cultures should be judged from their own point of view, not on the basis of how closely they adhere to our ideas.