What is what does it mean to knot someone?

Knots in the context you're likely referring to (often found in fiction, particularly in the realm of speculative erotica) are a feature found in certain fictional species or sometimes applied to humans in fictional settings. It's important to remember this is a fictional concept and has no basis in human biology.

The "knot" refers to a bulbous enlargement at the base of the penis of these creatures. During copulation, this "knot" swells and becomes temporarily lodged inside the vagina of the female.

The process of "knotting" someone refers to the act of this knot swelling inside the female, creating a temporary physical bond. This is often depicted as ensuring successful insemination and increasing the likelihood of pregnancy. It's usually portrayed as a pleasurable, although sometimes intense, experience for both parties involved. The knot typically remains in place for a period of time (minutes to hours depending on the fictional work) before subsiding.