Simple Definition:

Oligarchs are extremely wealthy individuals who have a significant amount of power and influence in society. They often control important industries and have a great deal of political influence [ability to shape and control government decisions].

For example, a group of oligarchs may own several major media companies [organizations that produce news and entertainment], giving them control over the information that reaches the public. They might also have close relationships with politicians, allowing them to shape government policies in their favor.

Very Simple Definition:

Oligarchs are very rich and powerful people who have a lot of control over important things like businesses and government decisions.

For example, a small group of oligarchs may own many big companies and be friends with important politicians.