The compromise appears to be in the language of conditional immunity. Here is how the language is spelled out.
The agreement would also pave the way for [telecom] companies ... to shed the nearly 40 lawsuits they face for allegedly participating in a prior version of the NSA program... To win immunity, they would have to pass review from a U.S. District Court.
...Critical to sealing the deal was a compromise that would grant conditional immunity to telecommunications companies for assistance they provided from September 2001 through January 2007. If the companies can show a federal district court judge "substantial evidence" they received a written request from the attorney general or head of an intelligence agency stating the president authorized the surveillance and determined it to be lawful, the cases against them will be dismissed.
Of course, telecom companies are always approached by the executive branch and frankly any telecom company that decides on its own to run surveillance should be prosecuted. As such, this is not much of a compromise. The Democrats caved in. In the meantime, they spent several months blocking this vital piece of national security legislation. After all their grandstanding, they ultimately arrived in the exact same place the President has always been in. Thus, one wonders why they needed to grandstand then.