A query variable is a proxy for the object(s) you are searching for when constructing a SPARQL query. SPARQL queries are composed of triples: a subject, predicate, and object. In SPARQL queries, each component of the triple needs to be either a query variable or a Uniform Resource Identifier (URI).
Query variables are indicated with a question mark or dollar sign followed by a word. The word you choose for a variable is arbitrary, but should be human-readable for ease of understanding, and must be used consistently within a query.
Lincoln (2015) “Using SPARQL to Access Linked Open Data”: In the following query, the variable is
?painting. This variable tells the triplestore to search for all values of
?paintingthat properly complete the statement
<has medium><oil on canvas>.
?paintingstands in for the node(s) that the database will return.
?painting <has medium> <oil on canvas> .