Ok I'll bite. The de minimus rule allows any import valued under $800 to enter the USA duty free. Chinese drop shipping companies and others leverage this rule to ship millions of packages direct to consumer. Many Chinese companies split a large freight shipments into dozens of smaller shipments to avoid paying duty fees.

My understanding is, trade war stuff aside, the effect this has on customs is to overwhelm their intake facilities with small packages, thus allowing shipments of narcotics and illicit goods to make it through without much chance of getting caught. The idea is that removing this rule will force shippers to bulk ship freight and allow customs to better inspect smaller shipments.