A commemorative speech is one where a person gives a speech praising the other person who, at times, has already died by having a profound influence and impact on others. And the main theme of this speech is to emphasize the 'values' laid or depicted by the person praised or talked about.
In giving a speech where he praised Martin Luther King and his leadership during the civil rights movement, Barack Obama brings attention to the work of King and his values, the values he laid down throughout his life. This is what constitutes a commemorative speech.
