In a nutshell, the telcoms were told by the administration to conduct wire-tapping ostensibly on terrorists. They complied, and now people like the ACLU wants to sue them for it. This bill specifically prevents that. It's essentially an amendment to a bill passed in the Carter administration as a response to the Watergate scandal to regulate domestic intelligence-gathering. My question is this: just what did you expect the telcoms to do? If you allow lawsuits against them, you're advocating a damned-if-you-do-and-damned-if-you-don't scenario. It's not like they could very well give the government the finger...and if you think otherwise, try it with the IRS next April 15th.
The right of the people to be secure in their persons, houses, papers, and effects, against unreasonable searches and seizures, shall not be violated, and no Warrants shall issue, but upon probable cause, supported by Oath or affirmation, and particularly describing the place to be searched, and the persons or things to be seized.
The whole issue of wire-tapping is debateable: you want to find out what terrorist activity might be going down, but you also don't want a slippery slope established for future invasive activity for other purposes. Ben Franklin was attributed to as saying, "those who trade freedom for safety deserve neither," but war-time activity has always been necessarily shady, such as the suspension of habeas corpus during and after the Civil War and the restriction of the press in WWII.
The qualifier, here, is the word I boldfaced in the amendment: "unreasonable" searches and seizures. To some, "unreasonable" is "any", while to another it's "harmfully invasive" and anywhere in-between and beyond. FISA doesn't do anything to spell it out, either, so the debate will (as it should) continue.