Randy Rider has put together an article for Officer.com that explains Garrity and how it should be used by the investigators and Officers. The write up is better and more simply than any other I have read.

Essentially Garrity breaks down to this: If you are being compelled to answer questions under fear of discipline, including termination, then these statements cannot be used against you in a criminal proceeding because they are coerced and in violation of your constitutional rights.

For the rest of the article check it out at Officer.com

For a more comprehensive explanation on Garrity read this article.