What does the title mean?

In the book, the horse Admiral is either the best steeplechaser in the country or nearly so. Therefore, in many of the races he runs, it is (practically) a certainty that he will win, or a "dead cert" that he will win. For those of you not familiar with the phrase, it is fairly common in English and US parlance to refer to things that are supposed to be a certainty.

The film does not play up Admiral's dominance of the field, though it makes him out to be a very good horse.