Dynamic fields allow Solr to index fields that you did not explicitly define in your schema. This is useful if you discover you have forgotten to define one or more fields. Dynamic fields can make your application less brittle by providing some flexibility in the documents you can add to Solr.
A dynamic field is just like a regular field except it has a name with a wildcard in it. When you are indexing documents, a field that does not match any explicitly defined fields can be matched with a dynamic field.
For example, suppose your schema includes a dynamic field with a name of *_i. If you attempt to index a document with a cost_i field, but no explicit cost_i field is defined in the schema, then the cost_i field will have the field type and analysis defined for *_i.
Dynamic fields are also defined in the fields element of schema.xml. Like fields, they have a name, a field type, and options.
LucidWorks recommends that you include basic dynamic field mappings (like that shown above) in your schema.xml. The mappings can be very useful.