Leviticus 18:12, "You shall not uncover the nakedness of your father’s sister: she is your father’s near kinswoman," is a prohibition against having sexual relations with one's paternal aunt (father's sister).

Here's a breakdown:

"You shall not uncover the nakedness of..." This is a common euphemism in Leviticus for having sexual relations with someone.
"your father's sister" This refers to one's paternal aunt.
"she is your father's near kinswoman" This explains the reason for the prohibition. Because she is closely related to your father, sexual relations with her are considered incestuous and forbidden.

In essence, the verse forbids a man from having sexual relations with his paternal aunt. This prohibition is part of a larger section in Leviticus 18 that outlines various forbidden sexual relationships, all of which are considered incestuous.

The rationale behind these prohibitions is often attributed to:

Maintaining family structure and order: These rules help define proper family relationships and prevent confusion regarding roles and inheritance.
Avoiding genetic issues: While not explicitly stated in the text, some argue that incestuous relationships were avoided due to the potential for genetic disorders in offspring.
Moral and ethical considerations: These laws were meant to define what was considered morally right and wrong within the Israelite community.
Preventing exploitation and abuse: These laws would have served to protect the vulnerable within families.

Different religions and cultures have varying interpretations and applications of these laws today.