... or a block of code (but since you context is Lego, probably the former)...

... and almost certainly if your general context is the Lego MindStorm product line, which features bricks with built-in microcontrollers for building 'intelligent' items such as robots.

see e.g.

Lego Mindstorms
From Wikipedia, the free encyclopedia

Lego Mindstorms is a line of Lego Group robot kits combining programmable bricks with electric motors, sensors, Lego bricks, and Lego Technic pieces (such as gears, axles, beams, and pneumatic parts) to build robots and other automated or interactive systems.

Lego Mindstorms is marketed commercially as the Robotics Invention System (RIS). It is also sold and used as an educational tool, originally through a partnership between Lego and the MIT Media Laboratory [1], [2]. The educational version of the products is called Lego Mindstorms for Schools, and comes with the ROBOLAB GUI-based programming software.

Lego Mindstorms may be used to build a model of an embedded system with computer-controlled electromechanical parts. Almost all kinds of real-life embedded systems, from elevator controllers to industrial robots, may be modelled using Mindstorms.

There is a strong community of professionals and hobbyists of all ages involved in the sharing of designs, programming techniques, and other ideas associated with Lego Mindstorms.
http://en.wikipedia.org/wiki/Lego_Mindstorms

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Note added at 1 hr (2006-10-21 20:12:39 GMT)
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another ref (you really should try googling Lego + "code brick")
Projects related to BusyBodies
The Lego Intelli-Train is a tangible toy train which reacts differently to ... "Code brick" can be snapped into the track to cause the train to toot, ...
snowedin.net/ideas/Projects+related+to+BusyBodies

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Note added at 2 hrs (2006-10-21 20:40:34 GMT)
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Hmmm, apparently not quite quite what I thought (and not that many relevant refs).

The following is from an e-bay site, so probably only available in google cache:

BRAND NEW BOXED LEGO DUPLO INTELLIGENT TRAIN SYSTEM (3335)

THIS IS AN EXCELLENT TRAIN SYSTEM THAT HAS SO MUCH INTERACTIVE PLAY.

FIRST YOU LAY THE TRACK 71CM X 59CM

THEN YOU ADD THE CODE BRICKS - BLUE CHANGES THE TRAINS DIRECTION,RED STOPS THE TRAIN FOR LOADING/UNLOADING CARGO (INCLUDED) AND THE PINK CODE BRICK ACTIVATES THE HORN.

THEN YOU ADD ONE OF THE TWO DRIVERS,THESE TRIGGER DIFFERENT SPEEDS AND HORN SOUNDS.

MY 3 YEAR OLD HAS ONE OF THESE AND CAN PLAY WITH IT FOR AGES, HE LOVES HEARING THE TRAIN 'START UP' ONCE IT HAS TO STOP AT THE RED CODE BRICK - WHICH HE HAS PUT ON THE TRACK! AND BY JUST PRESSING THE GREEN BUTTON ON THE TOP OF THE TRAIN IT ALL STARTS UP AGAIN.

PLUS IT YOU WANT YOU CAN ADD TO THIS AND LAY MORE TRACK OR LEGO BRICKS FOR BRIDGES ETC


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Note added at 2 hrs (2006-10-21 20:43:39 GMT)
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Presumably the or the train has some sort of intelligence that recogises a code from each different type of brick and causes the train to perform the corresponding action.

There are also google reference to 'code bricks' as components of interactive teaching systems that can be used to assemble physical systems that execute logical functions, but that apparently isn't what you're dealing with.

With all the greatest respects to the other answerers, this has little to do with Lego. It's techie jargon for programming code that is specifically identified or pointed out as waypoints or links for other programmers or computer programs. It also refers to the fundamental structure of a compúter program - the building bricks if you like.

Don't confuse with 'brick code'.

Please verify: -

I support your thought that the code brick can be something physical but provides virtual access support.

Because it "unlocks the product", it may be specially designed brick just to turn on the product and meant to give experience of the kind of swiping authenticated card when entering restricted area. The brick either has a code printed on it as a password to enter the web site, or displays the code when it is installed into the product.