This is the 3373rd most frequent Spanish word.
decidieron
"Decidieron" means "they decided."
Here, 'decidieron' is used as a main verb in the past to indicate that a decision was made by the subjects (They) in the past.
Ellos decidieron visitar la ciudad el fin de semana.
They decided to visit the city over the weekend.
In this sentence, 'decidieron' introduces a subordinate clause ('que el tema...sea medioambiental') which specifies the decision made.
Los estudiantes decidieron que el tema del proyecto sería medioambiental.
The students decided that the project's topic would be environmental.
In this context, 'se decidieron' is a reflexive form, meaning that Carlos and María settled on a choice, emphasizing their decision process.