A technique that splits data into related tables so each piece of data is stored in only one place. It reduces redundancy and avoids insert, update, and delete anomalies. First normal form (1NF) requires atomic values in each column.
In DP-900: storing several comma-separated phone numbers in one column breaks 1NF; a many-to-many relationship is modeled with a junction table. Analytics does the opposite (denormalizes) to reduce joins.
Database normalization is a concept that can show up on the exam. Check the full guide to see it in context.