Bulk fermentation is the period after bread dough is mixed and before it is divided and shaped. Yeast produces gas while the dough develops flavor and structure; some recipes include stretch-and-folds during this stage. Temperature, flour, inoculation and dough strength all affect its duration.
It is separate from the final proof after shaping. Use the recipe's signs of readiness rather than assuming every dough should double in volume.
Reference: Further reading.