Relevance of the title Pulp Fiction?

From Wikipedia :-

The film's title refers to the pulp magazines and hardboiled crime novels popular during the mid-20th century, known for their graphic violence and punchy dialogue.

From Wikipedia page on Pulp magazine:-

Pulp magazines (often referred to as "the pulps") are inexpensive fiction magazines published from 1896 through the 1950s. Pulps were printed on cheap paper with ragged, untrimmed edges.

The term pulp derives from the cheap wood pulp paper on which the magazines were printed.

So it's named Pulp Fiction because it is Pulp magazine kind of story.

Ankit's answer is 100% correct, but I also just wanted to add to it that the original title of the script took its name from a specific pulp magazine called "Black Mask" (apropos of nothing, the Roger Avary script which eventually became the Butch story-line was titled "Pandemonium Reigns"). I suppose there may have been a potential rights issue of some sort regarding this (despite the Jet Li movie which got released a few years later), and Tarantino and Avary decided to go with the more general title for the screenplay that ended up being the one we know.