This header is called set of change, or hunk. Each hunk starts with a line that contains, enclosed in @@, the line or line range from,no-of-lines in the file before (with a -) and after (with a +) the changes. After that come the lines from the file. Lines starting with a - are deleted, lines starting with a + are added. Each line modified by the patch is surrounded with 3 lines of context before and after.
An addition looks like this:
@@ -75,6 +103,8 @@
foo
bar
baz
+line1
+line2
more context
and more
and still context
That means, in the original file before line 78 (= 75 + 3 lines of context) add two lines. These will be lines 106 (= 103 + 3 lines of context) through 107 after all changes.
Note the difference in from numbers (-75 vs +103), this means that there were other changes in this file before this particular hunk, that added 28 (103 – 75) lines of code.
A deletion looks like this:
@@ -75,7 +75,6 @@
foo
bar
baz
-line1
more context
and more
and still context
That means, delete line 78 (= 75 + 3 lines of context) in the original file. The unchanged context will be on lines 75 to 80 after all changes.
Note that from numbers in this hunk are equal (-75 and +75), this means that either there were no changes before this hunk, or amount of added and deleted lines in previous changes are the same.
Finally, a change looks like this:
@@ -70,7 +70,7 @@
foo
bar
baz
-red
+blue
more context
and more
still context
That means, change line 73 (= 70 + 3 lines of context) in the file before all changes, which contains red to blue. The changed line is also line 73 (= 70 + 3 lines of context) in the file after all changes.