The series gets its name because of its unique concept: each card was blank until made wet (usually by a kid's own spittle) and a developer agent (the 'orange mystery paper') was applied, at which time the image would fade into view much like a traditional photograph. The backs of the cards identified the subset and card number, along with a quiz question about the subject on the front. As one can imagine, many existing card images weren't allowed enough time to develop properly by their eager new owners.
The baseball subset of this series can be considered the first baseball cards produced by Topps, until their 1951 series. The baseball cards in the Magic Photos series are the most expensive ones of the set.
The Topps 1956 Hocus Focus series would use a similar 'developing' process for its card images.