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.