The letter follows a similar push made by the tech giants previously and is mainly focused on the issue of transparency. These companies want to be allowed to disclose more information regarding government data requests.
Beyond that, the companies are pushing for improvements to "privacy protections" as well as for deeper and more "appropriate oversight and accountability mechanisms" to be put in place with regards to prying government eyes. At this point it appears these protections and oversight improvements are mentioned on a general basis as there is no information on what these measures could include.
While this sort of pressure has been in place previously, the open letter represents the tech giants coming together behind a bill that has received bipartisan support with sponsorship from Sen. Patrick Leahy (D-VT) and Rep. Jim Sensenbrenner (R-WI). The open letter, which was sent to members of the Senate Judiciary Committee, "applauds" the USA Freedom Act, which would see an end to bulk data collection of private communications of Americans.
Leahy and Sensenbrenner wrote an editorial piece regarding the importance of the bill saying that the intelligence community has not been able to justify its actions and that it's "simply not accurate to say that the bulk collection of phone records has prevented dozens of terrorist plots. The most senior NSA officials have acknowledged as much in congressional testimony." Senator Leahy added in a later statement that it is "time for serious and meaningful reforms so we can restore confidence in our intelligence community."
While both the companies and the government are obviously trying to restore public confidence, there is a more international factor at play here as well. With the recent news that Germany was considering blocking the country off in favor of a local national internet, there is likely some worry among the tech industry that foreign users will move to new or local services to avoid NSA surveillance.