There are waves of enemies of two types, one will chase you and the other are turrets in the scenario, the red bar around the ship shows you the closest enemy.
Once you kill all the enemies of the wave, a boss will spawn in the center, once again the red bar will show you where it is.
There is a total of 3 waves and 3 bosses.
If you have any other doubt or still confused about something I'll be happy to explain in more detail