Yes , Atlas stores information in HBase. For fast information retrieval , the data need to be indexed and retrieved when queried.
Atlas uses Janus Graph. Janus graph has 2 types of indexes : composite and mixed indexes.
Composite indexes are supported with primary storage backend (HBase in this case), mixed graph indexes require indexing backend for full text search , numeric range search etc., Here , Solr is used by Atlas as indexing backend.
You may read about Janus Graph for more detailed information.