Explanation of svn diff unified format

[Hussein Baghdadi]

Hi.
I'm reading "Pragmatic Version Control Using Subversion, 2nd Edition".
First we have this file:
---
Day.txt
monday
tuesday
wednesday
thursday
friday
---
The author then added two lines:
---
Day.txt
monday
tuesday
wednesday
thursday
friday
saturday
sunday
---
And to show the diff:
svn diff Day.txt

Index: Day.txt
===================================================================
--- Day.txt (revision 1)
+++ Day.txt (working copy)
@@ -3,3 +3,5 @@
wednesday
thursday
friday
+saturday
+sunday


Would you please explain to what does this line mean?
@@ -3,3 +3,5 @@
I can't follow the author.
Thanks.