You can agree to terms, but nothing will be signed until free agency officially opens. This also gives Franchises a chance to match offers, or players to see if their market value is higher or lower than what their team is offering them for renewal.
I think technically even verbal agreements are forbidden, though obviously that is unenforceable. When the lockout ended, there was a window like this where everyone agreed to deals and the media announced it, and then the deals were officially signed the first day of the new year.
The NFLN has like all next week set up with big blocks of time devoted to FA rumors and announcements, so more than anything I think they just want to preserve their television schedule
Albert Breer ‏@AlbertBreer
The NFL sent a memo to GM, head coaches and personnel directors this week on the tampering period.
The memo says club can "express its position as to signing bonus, length of contract, amounts of signing bonus and yearly compensation" ...
.. But clubs "cannot make an offer" or "enter into a written or oral agreement of any kind".