While it may look different in various databases, most provide you with the ability to set forth a series of rules about what you want to search within. Here are some common types of limiters along with what they do:
In the same way that IMDB.com tells you about a movie but doesn't give you access to the movie itself, the databases might have information to tell you about an item but not access to the item itself. You can tell the database that you only want items that you can access the item itself from that database by clicking on the "full text" limiter.
In some disciplines and for some topics it is important to use current information. You can tell the database to only give you results from a certain time period by limiting to a publication date range.
Library databases typically search a set of published works, but that doesn't mean those are always the types of publications you want, need, or are allowed to use. You can use the Source Type limiters to tell the database that you only want items from certain types of sources.
The subject limiter area is where you will find what tags are associated with the articles in your search results. Because these tags designate an article as being about that topic (rather than just mentioning the word) you can use these tags to tell the database that you only want articles tagged as being about that topic.