This statement is called the hunk header.
You compare two files using the diff command and this command records differences as groups of differing line and uses common lines to capture these groups within the files.
Such groups are called hunks of difference.
@@ -1,5 +1,4 @@
Here "-" denotes the source file and "+" represents the modified file to which the change refers to.
-1 represents the starting line from the source file
5 represents the length of hunk
+1 represents the starting line of modified or the to file
4 represents the length of hunk, i.e., the hunk is 6 lines long