Nightfall boost in Destiny 2 helps players complete high-difficulty Nightfall Strikes more efficiently. These services are typically offered by experienced players or professional teams who run the activity for you (or with you) to secure rewards like Ascendant Shards, Exotic gear, Enhancement Prisms, and Nightfall weapons.
Boosting can be done in self-play mode, where you actively participate, or piloted mode, where a pro player logs into your account to complete the run. This is especially useful for higher-difficulty levels like Master or Grandmaster Nightfall, where enemies are tougher, and powerful loadouts and strategies are needed.