The obvious answer is, it is the feeling that a higher Power can give and will be induced to give man what he needs and save him from trouble. This is true whether the Power is an invisible God or a visible man or image.
There must be in the first place power and willingness to use the power in the object worshipped, and as a counterpart, there must be a need, a fear, or other stimulus operating upon the worshipper to make him believe in the power and willingness of that power to render help.
"I draw to Me, My man from far off or even across the seven Seas to Shirdi, like a sparrow with a string fastened to its feet"-Shirdi Sai Baba, Indian Spiritual Guru-God 