Abstract and Subject are field limiters or filters that you can use to search more precisely or narrow results. 

ABSTRACT: An Abstract is a short summary of an article, often written by the author.  In the abstract the author will typically summarize their research question, methodology, and conclusions.

SUBJECT:  Subject Terms are a form of controlled vocabulary used in the databases.  A single subject term is applied to every article on a particular subject--no matter how the author refers to it--so that all the articles classified with that subject term can be found easily.

Our Field Searching guide has more information on which search fields work well with different search strategies.