"pleed the fifth" what does that mean?

This means that the person on trial is invoking their guaranteed right against self incrimination granted by the fifth amendment of the Bill of Rights.

The amendment states… No person shall be held to answer for a capital, or otherwise infamous crime, unless on 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

The reasoning behind this portion of the amendment is to prevent torture or blackmail from having any effect on the trials outcome. Both were problems and not uncommon before the Magna Charta was written… from which many of our guaranteed rights originate.