This editorial from the NYT, which Trump and his allies call a 'fake news" source, explains what the memo written by members of Devin Nunes', staff is all about. It is an effort to undermine the investigation of Donald Trump and his campaign staff by Robert Mueller. To accomplish that goal Nunes (R, White House) cherry picked data collected by the House Intelligence Committee, which he chairs, to argue that an "innocent member" of the Trump campaign was illegally placed under surveillance by the FBI. That claim is based upon the assumption that a FISA obtained by the FBI, in order to conduct the surveillance, was falsely obtained. Moreover, Nunes claims that the FBI was biased against Trump and that its investigation of the Trump surrogate was initiated by a dossier funded by the Clinton campaign (that is untrue). This editorial describes the process by which a FISA warrant is obtained which is very difficult to obtain under false documentation. It also shows that Trump's surrogate had a history of contacts with Russian spy's which prompted the FBI to watch him prior to the election campaign. Moreover, another member of Trump's campaign alerted intelligence sources in Australia about the surrogate during a drunken conversation. That information was forwarded to US Intelligence.
The Director of the FBI, who was appointed by Trump, expressed grave concerns about releasing the Nunes memo. He claimed that it exposed US sources of information and that it misused sources of information to reach its conclusions. Trump will ignore that warning because he is more interested in protecting himself from the ongoing investigation he is about exposing US intelligence sources. He is being supported by most Republicans in Congress for another reason. Information damaging to the Trump White House will be harmful in the 2018 midterm elections to GOP candidates. Above anything else they want to maintain control of Congress.