This is the 3rd most frequent Spanish word.
el
"The" (definite article).
In the first instance, 'el' serves as the definite article for the masculine singular noun 'perro,' meaning 'dog'. In the second instance, 'el' is the definite article for the masculine singular noun 'parque,' meaning 'park'.
El perro corre en el parque.
The dog runs in the park.
Here, 'el' is used before 'lunes,' which is a masculine singular day of the week, and 'museo,' which is another masculine singular noun.
El lunes vamos a visitar el museo.
On Monday, we will visit the museum.
In this example, 'el' is the definite article for the singular masculine nouns 'artista,' meaning 'artist' (implied), and 'mar,' meaning 'sea'.