This is the 51st most frequent Russian word.
мы
"We"
Here, 'мы' is used as the subject pronoun indicating the plural 'we' as the group of people going to the park.
Мы пошли в парк.
We went to the park.
'Мы' is used here twice, showing its role as the subject pronoun in two connected actions, providing cohesion in the narrative.
Когда мы увидели это, мы удивились.
When we saw it, we were surprised.
In this sentence, 'мы' is used in conjunction with 'с другом' to specify the group comprising the speaker and their friend.