In a cladogram, the outgroup is a species or a group of species that is closely related to the main group being studied but is not part of it. The outgroup is used to root the tree and provide a point of comparison. By comparing the main group to the outgroup, scientists can determine which traits are ancestral and which are derived.
For example, if you are studying the evolutionary relationships among mammals, you might choose a reptile as the outgroup. This is because reptiles are closely related to mammals but are not mammals themselves. By comparing mammals to reptiles, you can identify traits that are unique to mammals and those that are shared with reptiles.
Using an outgroup helps to ensure that the cladogram accurately reflects evolutionary relationships. It provides a baseline for understanding how traits have evolved over time and helps to avoid misinterpretations of the data.