Lic. 452151 (1984), Blue Knight Construction.
The lower the number, the longer the business has been established under that number.
When you hire a licensed contractor, it doesn't mean that they are a good craftsman, but at have you have a starting point.
Make sure to check the workers comp. A common trick some contractors use to avoid having it, is stating they have no employees on the CSLB site. In these cases, you would not have injury liability protection.
A wise consumer should make note of the license number when shopping for contractors. It's also a good idea to check it with the Contractors State License Board, for the following:
Date issued, Insurance, employees, Disciplinary actions, etc...
Here is a link to the Contractors State License Board.