Each building has a number of upgrades you can do, to improve its efficency.
The Level of your building represents how many upgrades you have completed. This makes it easier to see which building can still be improved, and which ones you have already improved to the max.

Possible upgrades are:

1. Unlock additional production lines
2. Increased production speed of production lines
3. Increased product storage of production lines
4. Increased retooling speed of production lines
5. Increased shippin area
6. Unlock additional parking space
7. Unlock additional resource options