In Roman Catholicism, a titular is a cardinal who holds a titulus, one of the main churches of Rome.
The concept of a titulus itself, some suggest, reaches back to antiquity when stones marked the confines of a property.
The concept of a "cardinal" (a word that has its origins from the Latin word for "hinge" referring to something or something of importance) comes in relation to those who made up the chief administrative body of the church within Rome; counsellors and assistants to and under the authority of the bishop of Rome -- that is to say the Pope.
In a nutshell then, the tituli represented the various centres of worship within the city of Rome and each were assigned their respective "cardinales" -- in modern terms, effectively the parish priest or archpriest.
So then, in summary, just as we today are familiar with the idea of a "cathedral" as a designation that denotes the location of the seat (i.e. cathedra) of the local bishop, or that a "basilica" as an honorific title bestowed on particularly beautiful and important churches (frequently but not necessarily cathedrals), so too is a "titulus" a title that comes with specific reference to the most important churches within the city of Rome, to which each is assigned a cardinal.