In Apache Solr's managed-schema.xml, it is an attribute that can be specified for field types and field definitions. The default is indexed="false", which declares that no inverted index will be created for that field, meaning searches will not be performed on that field.
For example, let's consider the schema design for product search on an e-commerce site. We want to include product name (product_name) and price (price) in the search, so we set indexed="true" for these fields. On the other hand, manufacturer name (maker_name, the name of the company that made the product) is not needed for search and is just a display item, so we set indexed="false", resulting in the following:
<field name="product_name" type="text_ja" indexed="true" stored="true"/>
<field name="price" type="pint" indexed="true" stored="true"/>
<field name="maker_name" type="string" indexed="false" stored="true"/>
