: Often stands for "DingXian" (a well-known developer/compiler of TWRP builds in the Chinese community).
The proper text for refers to a specific custom recovery file, likely for a Chinese-market smartphone (often Xiaomi or Redmi devices) . TWRP-3.3.1-0108-DX - PC.rar
If you are looking to flash this, ensure it is specifically made for your device's codename. Using a TWRP build compiled for a different model can "brick" your phone. TWRP-3.3.1-0108-DX - PC.rar