I've heard the term "brindle" used many times. What does it look like?
"Brindle" is a pattern of colors that is seen in the black, tan or brown colored area on a coat (see picture). You will not see it in the white portions. It is a repeating, wavy, tiger-like pattern of black or tan within those colored areas. It is a sign of foreign blood back in the dogs ancestry. For visual examples you can look up pictures of bull terriers, boxers, great danes and mastiffs in a dog encyclopedia. It is an unregisterable color and therefore the dog should not be used for breeding.
"Brindle" should not be confused with "grizzle", where black hairs are mixed with the tan.