A build number is a number that indicates the package installed on your application server or on your console.
This information is always needed when making a request on the Support portal.
Note that application server and console build numbers can be different as it is not always mandatory to upgrade the client console for each server upgrade.
Yet, it is highly recommended to maintain same versions. In some cases, an outdated version of the console can lead to errors.
To get it, open the console and go to Help > About.