How to boost relevance of Search API solr with the value of a certain field?
hook_apachesolr_query_alter($query) alter the query after it's prepared and cached.

As per example given in documentation which I found, you can use this hook in similar way to other query_alter hook. E.g -

function hook_apachesolr_query_alter($query) {
  //Adding one more filter.
  $query-&gt;addFilter("is_uid", 1);

  // Only search titles.
  $query-&gt;replaceParam('qf', 'label');
}


Find the documentation by clicking on the mentioned hook name by me. 

EDITED--

Sorry for giving example of other hook of apache solr module. You can find documentation of your hook here hook_search_api_solr_query_alter().

You can find a small snippet for example of this hook like --

function hook_search_api_solr_query_alter(array &amp;$call_args, SearchApiQueryInterface $query) {
  if ($query-&gt;getOption('foobar')) {
    $call_args['params']['foo'] = 'bar';
  }
}


Eric has wrote a nice article with example of this hook.