Shadari is a kind of metal object, which would look similar to the crown of the ancient kings. It would be placed on the heads of the devotees in Vishnu temples by the temple priests, after the darshan of the deities. It is a kind of blessing activity by the almighty to his devotees.
During the Treta Yuga, when Lord Rama went to the forest during his exile period, his brother, the great Bharata, had accepted the Padukas, Slippers of Lord Rama, which would be usually made out of gold metal or costly wood items. Bharata had kept the Padukas in the throne and successfully ruled the kingdom of Ayodhya, until the arrival of his brother Lord Rama. He has done so, since he considered the Padukas as equivalent to his brother Rama, and gave great respect to it. He has also done pujas to the Padukas, and also would worship the Padukas by considering it as his brother Lord Rama himself. He has kept such a great love and affection on his elder brother, and in this Kaliyuga, we cannot expect that kind of selfless devotion from most of the people.
Similar to the above incident, the Shadari is considered as the crown of Lord Vishnu, and by placing it on our head we can get the feeling of touching the great Lord Vishnu itself. Ancient Vaishnative saints while singing wonderful songs on Lord Vishnu, had mentioned about the greatness of the Shadari also.
Usually after giving Tulasi Teertham in the Vishnu Temples, Shadari would be placed on our head, since Tulasi is considered as an aspect of Ma Lakshmi Devi, and Shadari is considered as an aspect of Lord Vishnu. From this practice we can understand, that how the great Lord Vishnu has given more importance to his consort Ma Lakshmi Devi.
It is also believed that those who suffer from severe headache and prolonged illness are getting quickly recovered from their health issues after they consume Tulasi water and after the divine Shadari is placed on their heads, in the Vishnu Temples.