What exactly is Indium's ability to stabilize non-ferrous metals?
All I did was click on the reference from the History wikipedia link above for the sentence you were talking about. The sentence comes from a paragraph of a manuscript which is found here: http://pubs.acs.org/doi/abs/10.1021/ed011p270

Note the bolded part, that was what is meant. 

" Its[Indium's] identification as well as its name came from the beautiful indigo-blue line which it so prominently displays in its spectrum. Its commercial development came as the result of the knowledge that it should be a valuable stabilizer of non-ferrous metals much as chromium is a stabilizer of the ferrous metals. "



Above is quote from: 

French, Sidney J. (1934). "A story of indium". Journal of Chemical Education 11 (5): 270. Bibcode:1934JChEd..11..270F. doi:10.1021/ed011p270.