Carnal (cahr-NAHL, lit. carnal) means brother by blood. If one guy calls another broder (the mexicanized version of "brother") the speaker implies he feels a brother-like bond with that person. Carnal is the superlative of broder. You can have a lot of broders but only a few carnales. Use it when you mean it, for saying it in a casual fashion will make you sound like a boot-licking moron.
Cautionary note: Unfortunately, there is no comparable term to refer to such a tight bond between siblings of different sex or between sisters.
One of the most famous duos in old Mexican movies are Tin Tán and his carnal Marcelo, depicted below.