On June 21, 1891, Abbe Sauniere staged a procession through the village of Rennes-le-Chateau with a statue of the Virgin of Lourdes, which he then had installed on the Visigothic pillar in the garden of the church. The pillar that is presently there, depicted in the photo above, is actually a reproduction of the one that was once inside the church. All kinds of theories abound as to why he carved "Mission 1891" in the pillar, but it could simply be the commemoration of the event. It's not uncommon to put dates and names or other writings on structures when they are built or placed or remodeled.
Interestingly, the figure at the bottom of the pillar can depict either an entwined A and M, signifying "Ave Maria," or it might be Alpha and Omege, signifying "The beginning and the end".