Answer:
External political actors are the people or entities that could affect the outcome of our political situation but they do not belong in our political hemisphere.
For example,
Let's say that Country A has 2 candidates in a presidential election. The policies that planned by the first candidate will be beneficial for Country B while policies planned by the second candidate will do the exact opposite.
So, Country B sent its agents to mess with the election in Country A in order to help the first candidate win the election.
Under such circumstances, we can say that Country B is acting as an external political actors in Country A's politic.
