What does the following line from Romeo and Juliet mean?: "Then I'll be brief! O happy dagger, this is thy sheath. There rust and let me die."

The line
is from act 5, scene 3. After waking from her drug-induced slumbers,has just discovered the dead
body of her beloved . As one can imagine, Juliet is utterly devastated by this appalling . The
only way she can now be with Romeo is in death. There's nothing for it, then; she's going to
have to kill herself.

Juliet can hear the sound of a watchman and's page fast
approaching, so she knows she doesn't have much time to lose. That's what she means by
"Then I'll be brief." As for "O happy dagger," this refers to what is for
Juliet the fortunate discovery of Romeo's...