It's one of a number of common Spanish colloquialisms roughly equivalent
to English "Don't mention it" or, perhaps closer, "It was nothing". Nada
in Spanish means "nothing".
Another is "No hay de que" (sometimes further shortened to "No
hay"--pronounced like "no I"), which is probably probably a shorthand
for something like "No hay nada de que decir gracias"--fairly literally,
"There is nothing for which to say thank you."
Basically, "You're welcome."
Alan Beban
What does "De nada" mean?