This is the 597th most frequent Serbian word.
почели
"Почели" means "began" or "started" in English.
The word 'почели' is used here to mean 'started' and indicates the beginning of an activity (walking), performed by the subject (we).
Након кише, почели смо да шетамо.
After the rain, we started walking.
The word 'почели' refers to the initiation of work on a project, done by the subject (they) in this context.
Они су почели са радом на пројекту.
They have started working on the project.
In this context, 'почели' is used in an interrogative sentence, asking whether the subjects have commenced the activity (learning a new language).