This is the assertion that because a leader or celebrity is doing something, or believes something, that it must be good or true.
Appeals to authority are very useful in persuasion—they allow you to use the character and credibility of the authority as a proxy for your own character.
This is why celebrity advertising is so common—no one really cares what Coca Cola's marketing team has to say about Coke, but they do care what Selena Gomez has to say about it.
But to be legitimate arguments, appeals to authority should be backed up by reasoning and evidence.