The [link|http://www.fsf.org/licensing/licenses/gpl.txt|terms of the GPL] are a little difficult to understand in its native tongue, but to its credit it speaks a much clearer dialect of [link|http://en.wikipedia.org/wiki/Legalese|Legalese] than most software licenses. The license spells out the rights you are granted concerning the use and distribution of a program that uses it. Under its terms, you are allowed to examine and modify the program's source code... but if you distribute that code for any reason, you must make the modified source code publically available to whomever you distribute it to, free of charge.
In other words: if you change something for your own use, you are not required to make those changes available. If you change something and allow others to use those changes, you must provide them access to the source code so that they may do the same.