In Spanish, a grammatically correct sentence includes a personal pronoun (I, you, he, she, etc) and a verb. However, it’s very normal in formal and informal situations to remove the personal pronoun. Let’s take as an example the sentence “I go to the park with my dog.”

We can say “Yo voy al parque con mi perro” or most commonly: “Voy al parque con mi perro”. In the second sentence, the “yo” (I) is removed, yet the sentence maintains its sense and is also grammatically correct. 

Why does this work? 

In some languages like English, this couldn’t work because the verb doesn’t change much with the personal pronoun. In Spanish, however, each inflection is completely different because it changes per personal pronoun and whether or not it's in the past, present or future tense.

This way, even if we ignore the personal pronoun, we still know who we’re talking about. If I say “Vamos al parque con mi perro”, I’m referring to us (We go to the park with my dog).