Slicks is the colloquail term for magazines printed on paper having a more or less glossy finish, as opposed to pulps, the term used for magazines printed on pulp paper.
During the Golden Age, most magazines printing SF stories were pulps rather than slicks. SF authors aspired to be published in slicks, as they paid considerably more per word (20-50 cents/word compared with 5 cents/word) and since they were very choosey markets which were not SF genre markets, were considerably more prestigious. (From its earliest days, fandom yearned to be accepted by mainstream society (see "It is a proud and lonely thing to be a fan") and sf in the slicks was seen as a beginning.
Also involved: - Bradbury: An Illustrated Life - Don Ford - Slick