This module is now deprecated and will not be further maintained. If you're looking for an alternative for Drupal 6, check out Auto Index.
By default, Drupal indexes node content only when running the cron. This means that your new or updated content may only appear in search results after a few minutes, when cron has run. Using Instant search, you can make Drupal index your new nodes when submitting them (inserting or updating), after which they will instantly appear in search results.