What does it mean to say that the Catholic Church is visible and has marks?
The typical Protestant conception of the Church is that it is invisible. Though individuals may group together for fellowship and Bible study, their churches are really like clubs in a city. The real church, say Protestants, is the broad and unseen group of the saved.
The Catholic Church, in contrast, teaches that the Church is a visible organization. Being a visible organization, it can be identified–it has marks. The marks are that it is one, holy, catholic, and apostolic–“one” in that it is a unified organization, “holy” in that it is an organization divinely established, “catholic” in that it is to embrace all of mankind, and “apostolic” in that a line of succession has been kept with the authority Christ passed to Peter and the apostles.