Appropriate use of the word "modulated"
In this context, the 38 KHz signal is considered to be the carrier signal.  The detector will change state whenever it sees the appropriate light frequency that is chopped at that frequency.

The modulation that they are speaking of is turning that 38 KHz carrier off and on.  This modulation is what carries the information being transmitted to the receiver.

So you have 3 things in play here: 

1) the photo detector inside the receiver module wants to see a specific wavelength of light (normally IR) 

2) the electronics inside the receiver want to see that light turning off and on at a specific frequency (38 KHz carrier) 

3) the data that you want to transmit turns the carrier off and on according to the data actually being sent.  This modulation causes the output pin of the receiver module to represent that data.