DDR4 (Double Data Rate 4) is the memory standard most laptops sold in the last several years use. Compared to the older DDR3, it is faster and uses less power for the same amount of work, which also helps battery life.
On a refurbished laptop, DDR4 is a good sign of a reasonably modern machine; how much of it you have (8 GB, 16 GB, ...) matters more day-to-day than the DDR4 label itself.