Seed as bodily seed
Here’s what it means
In a bodily-purity context, 'seed' refers to semen or a reproductive emission.
Why the Bible says it this way
Elsewhere the same word often means descendants; those passages are explained under the separate descendant expression.
The Bible sometimes uses gentler words for a private or sensitive subject.
A helpful detail: This entry is limited to the bodily sense in Leviticus 15:16.