db is not an instruction from 8052 is an instruction for the assembler and may be different depending of youe assembler.
db put the folowing valye directly in the program memory.
The meaning of "Hola a todos" is "Hello to everyone" in spanish. And is only a string to test the routine, you must end the string with 0 value. After the string your code can continue normally.

I have problems with missing contrast too, after the right conection the problem was end.