This is the 739th most frequent Spanish word.
atrás
"Behind" or "back."
In this sentence, 'atrás' is used as an adverb to indicate direction, specifically referring to looking toward the area behind.
Mira hacia atrás para ver el paisaje.
Look back to see the landscape.
Here, 'atrás' functions as an adverb indicating a location, describing that the backpack was left in a place that is considered behind in reference to the current position.
Dejé mi mochila atrás en el aula.
I left my backpack behind in the classroom.
In this sentence, 'atrás' is used metaphorically to refer to time, specifically indicating delay or being behind a schedule.