The solr search engine runs an indexation process on each document added to a collection for the first time, and also every time it is updated. This process analyzes the content of each of the fields in the document, splitting the values of text fields into tokens, etc., according to the document structure defined in the schema.xml configuration file.
Normally, this process needs to carried out only once for every new or updated document. But sometimes it might be necessary to re-generate the index for the whole collection, for instance when schema.xml is edited to modify the structure of the document. This post explains how to carry out this re-indexation operation, and some considerations to be taken into account.