Many search tools give the the options to specify where in the results you would like your search words to appear.
The most common fields shared by most databases are: title, author, publication name and publication year. The default selection is often to search in the complete bibliographic information, this can be called for example "topic", "all text" or "all fields".
Example of how it may look when you select a field to search in:
In addition, each database often have their own specific fields that you can use. These field often include one or more fields for searching in the controlled vocabulary (thesaurus).
The database BIOSIS Previews in Web of Science have an additional 16 fields that are available from a drop down menu in the basic search, for example document type, language and taxonomic data. If you know the specific field codes (found here) and are comfortable with using the advanced search field, you can search in about 50 different fields in total.
Field search can be very valuable if you have interest in a fairly specific subset of publications, for example publications published in a certain country (author address field), publications dealing with a certain gene (gene name data), or publications dealing with a specific group of organisms (taxonomic data or taxa notes field).