Horizontal (or lateral) gene transfer just means that many life forms trade genes around, via pollen, for example, or ingesting something.
The prevalence of horizontal gene transfer is a blow for Darwinism because it obviates the need to explain how “it could have happened by Darwin’s mechanism.” The point is, it didn’t happen by Darwin’s mechanism.
And if that didn’t happen by Darwin’s mechanism, maybe much or most of what is claimed for Darwin’s mechanism didn’t happen that way either.
An important strategy of Darwin lobbyists has been to create a narrow mental track, along the lines of “How would Darwin explain this?” And science writers reverently obey. It turns out there is a highway on the other side of the hedge.
See also: Never mind horizontal gene transfer, now the rap is horizontal gene THEFT
Darwinists try to come to terms with horizontal gene transfer
Horizontal gene transfer: Gene from bacteria lets beetle feed only on coffee beans