"Eles se apresentaram no palco principal."

Translation: They performed on the main stage.

[ppmcltx]

so, apresentar-se = perform?

[Paulenrique]

yes. that's a reflexive verb...

[Johnni0]

so it could never be "they presented themselves"?

[Paulenrique]

that's also correct ;)

[iGerebrum]

"Apresentar" is the verb to present. "Acuar" is the verb to perfrom/(execute an) action.

[BobSK1]

They appeared on the main stage.
To appear on stage means to perform in English-language theater.