Solr is a popular enterprise search platform built on Apache Lucene. It’s reliable, scalable, and fault tolerant. Read more about it here.
Although Elasticsearch is the default search engine that ships with Liferay DXP, it’s perfectly valid to use Solr instead. In particular, if you’ve already been using Solr with a previous version of Liferay DXP, or your platform (for example, your OS or JVM) isn’t supported by Elasticsearch, you might choose to use Solr to search and index your Liferay DXP data.
There are circumstances that force you to use Elasticsearch instead of Solr. Read here for more information.
Liferay DXP 7.1 supports Solr 7.5.x through the Liferay Connector to Solr 7 application.