Hey, Jose!

Originally Posted by: joseluiscamusrI wander why in the first lesson of Rock Style, the A chord have a note in paréntesis.

Notes in parenthesis indicate they are lightly muted, or not necessarily meant to be played but sometimes ring sypathetically a little anyway. In this case Anders tells you you can use that note or avoid it to make the A chord less complex & potentially muddy because that (2) is the major 3rd of the A chord & with an overdrive tone it could get muddy.

https://www.guitartricks.com/lesson.php?input=19318&s_id=1487

So you can lightly mute the string to create the muted ghost note at the 3rd fret. Or you can avoid hitting it at all.

Also, sometimes parenthesis indicate a note held over from a previous note.

Hope this helps!