A Marian apparition is an event in which visions or images of Mary (the mother of Jesus) allegedly appear in modern times. Note, a religious person may see an image of Mary when a non-religious person would see nothing or would see any woman's image, perhaps the heroine of a romantic novel.
These can be visions claimed by one person, appearances alleged to have been seen by large numbers of people in a particular place, rumors that an image of Mary is appearing on a particular rock formation or shadow at a certain time of day, etc. Some of these claimed sightings of Mary are forms of religious pareidolia in which "Mary" is seen in an ordinary object.
In the United States, the visions of Mary by Veronica Lueken in Bayside, New York from 1970 to her death in 1995 were heavily promoted with billboards and a 1-800 number you could call for a packet of brochures on Mary's message. In this case, Mary's message as channeled through Veronica Lueken was heavily infused with apocalyptic warnings of a coming New World Order conspiracy.
In 2004 a woman put up for sale a 10-year old grilled cheese sandwich which she said had an image of the Virgin Mary on it. It fetched $28,000.00 on eBay despite having a bite taken out of it. Eternal
salivation salvation is tempting, but damn, you just can't get enough melted cheese.