tax_query is a 2-dimensional array, each sub-array can be considered to be a taxonomy query, typical its of the form:
array(
'taxonomy' => 'people',
'field' => 'slug',
'terms' => 'bob'
)
(e.g. posts in with the term with slug ‘bob’, of the ‘people’ taxonomy).
The fact that tax_query is a two dimensional array means that you can have multiple taxonomy queries. In this case you may want to return posts which match all taxonomy queries (relation => 'AND' ) or posts which match at least one taxonomy query (relation => 'OR' ).
As the codex:
states
This construct allows you to query multiple taxonomies by using the relation parameter in the first (outer) array to describe the boolean relationship between the taxonomy queries.
For example:
$args = array(
'post_type' => 'post',
'tax_query' => array(
'relation' => 'AND', //Must satisfy all taxonomy queries
array(
'taxonomy' => 'movie_genre',
'field' => 'slug',
'terms' => 'action'
),
array(
'taxonomy' => 'actor',
'field' => 'id',
'terms' => array( 103, 115, 206 ),
'operator' => 'NOT IN'
)
)
);
$query = new WP_Query( $args );
returns all posts with move_genre term with slug ‘action’ AND does not have the actor terms with IDs 103,115,206.
On the other hand (notice relation change):
$args = array(
'post_type' => 'post',
'tax_query' => array(
'relation' => 'OR',//Must satisfy at least one taxonomy query
array(
'taxonomy' => 'movie_genre',
'field' => 'slug',
'terms' => 'action'
),
array(
'taxonomy' => 'actor',
'field' => 'id',
'terms' => array( 103, 115, 206 ),
'operator' => 'NOT IN'
)
)
);
$query = new WP_Query( $args );
returns all posts with either have move_genre term with slug ‘action’ OR do not have the actor terms with IDs 103,115,206 (or satisfy both).