Leviticus 15:20 deals with ritual impurity caused by a woman's menstrual cycle. The verse specifically states: "‘Everything that she lies on in her impurity shall be unclean. Everything also that she sits on shall be unclean."

Here's a breakdown of what that means within the context of the ancient Israelite society and religious practices:

Impurity, not Sin: It's crucial to understand that the "impurity" mentioned here isn't about sin or moral failing. It is a state of ritual uncleanness, akin to being in a state unfit to enter the Temple or participate in certain religious activities. It's like contamination, but it’s specifically ritually.
Menstrual Flow: The chapter describes a woman's menstrual flow as a source of ritual impurity.
Contact Contamination: This verse outlines how ritual impurity could be transmitted through contact. Anything a woman in her state of impurity lay or sat upon became ritually unclean.
Scope of Uncleanliness: The uncleanliness isn't permanent. The following verses in Leviticus explain how people or items that came into contact with the ritually unclean person or objects can be purified, usually through washing and a waiting period.

Reasons for These Rules (Interpretations):

There is not a consensus of why these laws existed, but there are some common theories:

Hygiene: Some scholars suggest these rules might have indirectly promoted hygiene practices in a time when knowledge of germs and disease was limited. Separating a menstruating woman could have helped reduce the spread of potential infections.
Ritual Purity and Sacred Space: The primary focus seems to be about maintaining the purity of the Tabernacle/Temple and sacred spaces. Menstruation was considered a process associated with life leaving the body, and contact with it was seen as incompatible with the holiness required for approaching God.
Respect for the Sacredness of Life: Some scholars interpret these laws as a way to recognize the life-giving capacity of women and the unique physical processes of the female body. Separating a menstruating woman could be seen as a way of acknowledging the sacredness of her body.
Social Order and Boundaries: The laws could have served to reinforce social roles and boundaries within the community.

Modern Interpretations:

Many modern Jews and Christians do not adhere to these specific purity laws, understanding them as specific to the historical and cultural context of ancient Israel. They may still find value in the principles of respect for the body and the sacredness of life, but interpret them differently in contemporary society.

In summary: This verse from Leviticus describes how ritual impurity was transmitted in ancient Israel, specifically concerning a woman's menstrual cycle. It is important to understand the context and interpret the laws within their historical and cultural framework. It is also important to recognize the difference between ritual impurity and sin.