Fine-grained DML (Data Manipulation Language) is a feature in BigQuery that allows users to optimize the execution of UPDATE, DELETE, and MERGE statements on tables. It does this by:

1. Optimizing mutations. Fine-grained DML optimizes mutations at a more detailed level than the previous file-group approach.
2. Reducing data rewrites. Fine-grained DML aims to minimize the amount of data that needs to be rewritten during operations.

To enable fine-grained DML, users can:

Set the enable_fine_grained_mutations table option to TRUE when running a CREATE TABLE or ALTER TABLE DDL statement.