Wiktionary
n. (context idiomatic English) Assistance given to a person or cause, usually without getting directly involved.
Wikipedia
Moral support is a way of giving support to a person or cause, or to one side in a conflict, without making any contribution beyond the emotional or psychological value of the encouragement.
For example, in a war between two countries or alliances, a third nation may give moral support to one side, without actually participating in the conflict (for example, Paraguay in World War II).
Another common example can be found in sports. By coming out to watch one's friend's team play a match, one is likely not directly supporting their team in any significant way (especially if there is no charge to attend), but one's friend may still feel encouraged by the moral support of one's presence.
The line between moral support and other forms of help is often hard to draw. For example, some athletes report that they play better when the spectators encourage them—and in some cases referees' decisions may be influenced by a partisan crowd.
There is also moral support that one can offer someone who is experiencing a difficult situation. One may not be able to offer any concrete assistance except empathy.
Usage examples of "moral support".
When he had reported himself to the commander of the regiment and had been reassigned to his former squadron, had been on duty and had gone out foraging, when he had again entered into all the little interests of the regiment and felt himself deprived of liberty and bound in one narrow, unchanging frame, he experienced the same sense of peace, of moral support, and the same sense being at home here in his own place, as he had felt under the parental roof.
And in Rome, providing moral support for the plot, was the pope, Sixtus IV.
But I'll come down anyway in a little while to lend moral support.
But she obviously has a great deal of influence on Oedipus, and he looks to her whenever he needs advice or moral support.