Ikea led table guide

The following codes are excuted in Arduino IDE.

Serial.println('abcd', DEC);    //shows: 25444 in decimal base
Serial.println('abcd', HEX);    //shows: 6364 in hex base

How to relate 0x6364 with 'abcd'? Any hints would be very helpful.

In fact, what is the meaning of 'abcd' in Computer Science?