What does this jQuery code with square brackets and a caret mean?

Question

What does this jQuery code mean?

$('tr[id^="message"]')

Answer 1

It means - select all tr elements which id attribute starts with message string

http://api.jquery.com/attribute-starts-with-selector/

Answer 2

[] refers to an element's attribute(s) (id being one) and id^ is a wildcard which means the id must start with "message".