Per Investopedia: Definition of shelf registration: "A regulation that a corporation can evoke to comply with U.S Security Exchange Commission for a new stock offering for up to three years before doing the actual public offering....", "Sometimes current market conditions are not favorable for a specific firm to issue a public offering...By using shelf registration, the firm can fulfill all registration related procedures beforehand and go to market quickly when conditions become more favorable."
I think that NLY is preparing to come out of its hibernation. A little skinnier perhaps, but a sign of life again.
There is no sign that AGNC has used the shelf registeration yet, and AGNC did a normal secondary offering a few weeks ago. I don't think NLY filing the shelf registeration means that they will use it soon, it just gives them the option in the future should the choose to use it.