DDRAM stands for Display Data RAM.
The Display Data RAM holds the letters that get shown on the LCD of a character LCD module. For instance the letter ‘A’ is stored in its ASCII equivalent 65DEC (0x41HEX) in the DDRAM.
So a 20×2 character LCD would have enough DDRAM to store 40 letters. The value in the DDRAM is used to find the correct bitmap in the CGROM (Character Generator ROM) or CGRAM (Character Generator RAM), it is this small bitmap that gets displayed on the LCD.
This term very likely has its beginning in the granddaddy of all LCD controllers, the venerable Hitachi HD44780.