What is "supplicatio"

In ancient Roman religion, a supplicatio is a day of public prayer when the men, women, and children of Rome traveled in procession to religious sites around the city praying for divine aid in times of crisis. A supplicatio can also be a thanksgiving after the receipt of aid. Supplications might also be ordered in response to prodigies ( prodigia); again, the population as a whole wore wreaths, carried laurel twigs, and attended sacrifices at temple precincts throughout the city.