RDBMS, also known as Relational databases, are structured and easy to query with a language like SQL, but they have limitations when it comes to unstructured data. And scaling usually means buying far more servers and even then , that has limitations.
Not all data, however, is that easily organized. Semi and Unstructured data like IoT sensor data, social media,. photos, videos, location-based GIS information, web or mobile activity, and usage metrics can’t be neatly broken down nor should be, but nobody wants a data swamp.
Things like Hadoop, HBase, Cassandra, MongoDB and other NoSQL or NewSQL like databases trade tables for documents or json or blocks or columnar like schemas and more.