what do they mean when a database is “flattened out” vs. normalized?
"Flattened out" typically refers to a database where you have a single (or few) very large tables.
"Normalized" refers to whether the data has been organized into well structured, related tables. This typically reduces duplication of values across rows in a table by pulling the values into a separate table, and relating to it by ID.
For details, see Database Normalization.