It only appears on the persons page in the database, but it is used in the census indexes to cross-link to the database. In general, it doesn't mean a thing. Here is the real tip here. Go to the search page, type in the number with the I - i.e. I12509, click search, and the options should be limited to just the one person. People with small numbers - like me, I1, will not work well.
Big numbers work very well.
You can also book mark a person's page if that is where you always go in the database. I am always looking for ways to direct people to this site. A reciprocal link can be made on my site if you will let me know. You can add a banner to your link if you choose. Visit the Banners Page and choose from the graphics displayed there.