Post revisions are a WordPress feature that allows you to undo changes and go back to an earlier version of your posts and pages, it works by auto-saving every 60s while you edit content.
Since revisions can take up a lot of space in the database, it is a good idea to delete them.
DELETE FROM wp_posts WHERE post_type = "revision";
DELETE a,b,c FROM wp_posts a LEFT JOIN wp_term_relationships b ON (a.ID = b.object_id) LEFT JOIN wp_postmeta c ON (a.ID = c.post_id) WHERE a.post_type = 'revision'
define( 'WP_POST_REVISIONS', 10 );
Replace 10 with the number of revisions you want to keep for each post.