That means that he is leading the race.

It's not an explanation, per se, but in English it's equivalent phrase would be 'Geer Wilders is at the head of the pack.' Instead of an animal, like Guillaume explained, we think of people in a race as a pack, or herd, of animals. The opposite is to be 'at the back of the pack'.

Pour compléter la réponse d'Olivier, on dit "faire la course en tête" ou tout simplement "être en tête". Au contraire si l'on est dernier, on dira (pour le cyclisme seulement) "être en queue de peloton". On a la tête et la queue du peloton de cyclistes, comme si c'était un animal.