Subject indexes provide a system of controlled vocabulary that can be useful in identifying ideas that may have several common terms, or in eliminating search results that contain a keyword, but are not primarily about that keyword.
You are looking for information on mosaics. To find journal articles you access the Art Abstracts database and type mosaics into the Basic Search box. Over a thousand results come back, covering everything from "Television and the unconscious: Donald Kuspit, an interview," where the abstract refers to "Television is disintegrative if for no other reason than that the image is inherently broken up much like a mosaic," to an article on "Barbie's Dream House." A scan of the subject index in Art Abstracts reveals that there are (at this time) 17 articles about mosaics in general; 9 articles about mosaics, american; 43 articles about mosaics, byzantine, etc.
Many databases provide subject indexing for more sophisticated searching. Library of Congress subject headings and the Getty Art & Architecture Thesaurus are two examples of subject indexes. To take best advantage of the searching power, it is important to understand and use the available subject headings (also called descriptors.)