MLA stands for Modern Language Association. The Modern Language Association publishes a handbook, called The Handbook for Writers of Research Papers, that dictates the format and style of papers published in the humanities. It includes formatting rules for the layout of papers and for citations within and at the end of papers. It also has rules for the use of footnotes and endnotes and guidelines for puncutation and grammar.
The MLA's guidelines are important because they promote consistency within a discipline. Everyone writing history papers, for example, uses the same rules. So everyone reading a history paper knows where to find citations, how to read tables, when to look for a footnote, and so on.
There are other guidelines as well. Two of the most common, aside from MLA, are APA for the sciences and Chicago for journalism. All of these guidelines are updated periodically to make allowances for changes in media. For example, the previous edition of the MLA guidelines did not have any rules for citing websites, but the latest edition does.