The Pontifex maximus was the leader of the pontifical college (collegium pontificum), the highest priestly order in the Roman state religion.
One member of the pontifical college was co-opted by the college as the leader, hence the title pontifex maximus. Initially only one of the three flamines maiores could be pontifex maximus, but towards the end of the Roman Republic the rules were changed. After 104 BCE the pontifex maximus was elected in a sort of limited popular elections between candidates acceptable to the college, and all the pontifices were eligible. The title was held for life.
The pontifex maximus had taken over many religious duties from the rex sacrorum who had inherited them from the kings. Among these duties were the administration of the Roman calendar and the clock. Until the arrival of the first reliable sundial the time of day was announced by the herald of the pontifex maximus at midday and evening, based on observations of the sun from the steps of the Curia Hostilia in the Comitium.
The official residence of the pontifex maximus was the Domus Publica in the Forum Romanum, and his religious duties was carried out from the Regia. The Vestal Virgins sorted under the pontifex maximus.
The last to hold the title of pontifex maximus under the Roman Republic were Julius Caesar (62-44 BCE) and M. Aemilius Lepidus (44-12 BCE). After Lepidus the title passed to Augustus, and henceforth the title of pontifex maximus would be a prerogative of the emperor. It remained so until the 5th century when it passed to the Pope, who have used the title ever since.
The title pontifex means literally “bridge builder” because they bridged the gap between the world of man and the world of the gods.