This refers to the prohibition found at 19 U.S. Code § 1305
(a) Prohibition of importation
All persons are prohibited from importing into the United States from any foreign country [...] any obscene book, pamphlet, paper, writing, advertisement, circular, print, picture, drawing, or other representation, figure, or image on or of paper or other material, or any cast, instrument, or other article which is obscene or immoral...
What constitutes "obscene or immoral" is subjective and determined on a case-by-case basis, for example by applying the Miller test, being:
the United States Supreme Court's test for determining whether speech or expression can be labeled obscene, in which case it is not protected by the First Amendment to the United States Constitution and can be prohibited.
The Miller test has three "prongs" which have to be satisfied for the material to be considered obscene:
1 Whether "the average person, applying contemporary community standards", would find that the work, taken as a whole, appeals to the prurient interest,
2 Whether the work depicts or describes, in a patently offensive way, sexual conduct or excretory functions specifically defined by applicable state law,
3 Whether the work, taken as a whole, lacks serious literary, artistic, political, or scientific value.