To plead the 5th means to exercise the right to not give testimony under oath which could incriminate them. Here is the text of the amendment: "No person shall be held to answer for a capital, or otherwise infamous crime, unless on a presentment or indictment of a Grand Jury, except in cases arising in the land or naval forces, or in the Militia, when in actual service in time of War or public danger; nor shall any person be subject for the same offense to be twice put in jeopardy of life or limb; nor shall be compelled in any criminal case to be a witness against himself, nor be deprived of life, liberty, or property, without due process of law; nor shall private property be taken for public use, without just compensation."
When someone says I pled the 5th they mean the 5th amendment. Which gives you the right not to talk (usually when you are in trouble) and I think it also gives you freedom of speach, but Im not sure about the last part.
It means I plead the 5th amendment, which basically means that have the right to not talk because it could be used against you in court. In your regular life you could say that if, for instance, someone asked you which of your 2 best friends was your favorite. You could refuse to answer and say I plead the 5th :-)