NI -  I assume stands for Needs Improvement.  Everyone needs improvement in some area.  No one is perfect.  It is something to use when setting goals.  In our performance review system, NI is taken as constructive criticism.  We must always seek continuous improvement in all areas.

Example:  Attention to detail - NI When goal setting, strive for improvement.  Implement a process whereby you highlight key items in messages or proof all correspondence before sending. (This is only an example)  Another goal is to take a training class over the next quarter or six months on improving details skills, proofreading or improving listening skills, whatever the case may be.  

Do you know exactly what the NI is in yet?  HR is probably following up to make sure the process is being followed and the developmental need is being properly communicated.  If you are called in and verbally counseled for performance infractions or counseled, (warned) in writing, then I would be worried.  

Take it as constructive criticism during your review and when discssing it with you boss during the review, be positive and take it for what it is.  Room for improvement.  Let us know how it turns out.