NBI stands for New Building Installation. Brokers sometimes use this term to describe a space that has never been occupied before.
There are pros and cons to “new” space. The pros are that you don’t have to demo someone else’s partitioned space, you’ve got a clean slate.
The cons are the extra costs involved to run wiring, build interior walls, doors, flooring, painting, etc. However, the landlord will usually contribute or pay for these costs, depending on the length of the lease you negotiate.