The codes unlock extra content for the game (at the time only 5 vehicles) or all windows or bricks.

To activate a code you have to load your savegame, now you are at the screen where you can join your world etc. , now you have to klick on the 4. icon from the left side.
Now a field appers where you can enter the codes.
(if you use a controler you can enter them also with your keyboard its much faster)
Then you confirm it, and you'll get the confirmation that the code unlocked something.

To use them you join your world and enter the menu to spawn in something,
look for the unlocked things and you can "buy" them for free.

Vehicles:
P42FJ6 Police Car (LEGO City)
BG7DWK Getaway Car (LEGO City)
U98BR2 Pizza Van (LEGO City)
XP3BN2 Lance’s Driller (NEXO KNIGHTS)
LY9C8M Lock & Roller (NEXO KNIGHTS)
ND284C Jungle Buggy
PPA72V Police Tuk Tuk
VN4MHZ Barbarian
TJPZLV Chinese New Year Dog
NA3ZKE Toy Soldier
8B3VEQ Pink Convertible
XP76VF Jungle Cargo Helicopter
X29VTY Monster Scientist (Monster Pack)
Other:
BR1CK5 unlocks all bricks for building
F1XTRS unlocks all doors and windows

If you have any questions just ask me down in the coments.