Hi Glenn, sponsor units are apartments that are still owned by the original developer (i.e. sponsor). They've typically been rented out over the years.
Since it's a sponsor unit, you won't have to go through board approval or need an interview. You just need to be financial qualified, and the qualifications can be less stringent vs a normal apartment in the co-op.
Another interesting note, I've heard that you may be able to keep it as a sponsor unit when you buy, in order to continue subletting indefinitely. Has anyone else heard of this?
Lastly, definitely learn what closing costs are before you proceed (https://www.hauseit.com/closing-costs-nyc/) as they can be quite significant in NYC.
You should also look into getting a broker commission rebate from a reputable local company like Hauseit. It's an easy way to cover the cost of the Mansion Tax (1% of sale price for purchases over $1 million)!