This is from the NFL's CBA - which can be found here:
http://www.nflpa.org/Members/main.as...Complete#art16
College Draft:
Section 11. Undrafted Rookies: Any person who has not been selected by a Club in a College Draft shall be free, after the completion of a College Draft for which he is eligible, to negotiate and sign a Player Contract with any Club, and any Club shall be completely free to negotiate and sign a Player Contract with any such person after such date, without any penalty or restriction, including, but not limited to, Draft Choice Compensation between Clubs or First Refusal Rights of any kind.
In simple english, any player wishing to join the NFL must first be subject to the draft. If they are not drafted, then they are free to sign with whomever they choose. If you are not eligible to be drafted, then you cannot play in the NFL.