Muggles' Guide to Harry Potter/Magic/Howler

Overview
Howlers are messages that are carried by Owl Post, like letters. They are distinctively red-colored, and are used to carry urgent and angry messages.

Extended Description
Howlers appear several times in the series. The first one we see is sent to Ron by his mother in Harry Potter and the Chamber of Secrets. The next is sent to Neville by his grandmother in Harry Potter and the Prisoner of Azkaban. Several appear in Harry Potter and the Goblet of Fire as hate mail sent to Hermione, due to Rita Skeeter's article about her. Finally, one is sent to Harry's Aunt Petunia in Harry Potter and the Order of the Phoenix.

A Howler is not used for just any message, as true to their name, they are extremely loud – according to the text, a hundred times as loud as one's natural voice, though that could be simply the apparent effect. Additionally, if not opened promptly, they tend to explode: though we do not see that happening, Nevile Longbottom advises Ron that it is better to open one immediately and take the amplified scolding, rather than leaving it unopened and letting it explode; and Percy comments in Harry Potter and the Goblet of Fire that exploding Howlers have been scorching his desk.

Questions

 * 1) What is the meaning of the howler received by Aunt Petunia?