Backslash zero delimiter '\0'

It's the null character; more info in this Wikipedia article.

The two-character \0 representation is used in C source code to represent the NUL character, which is the (single) character with ASCII value 0.

The NUL character is used in C style character strings to indicate where the end of the string is. For example, the string "Hello" is encoded as the hex bytes:

48 65 6c 6c 6f 00

In this case, the C compiler automatically adds the 00 byte on the end of any double-quoted string. If you wrote the constant as "Hello\0", then the C compiler would generate:

48 65 6c 6c 6f 00 00