While you write a post in WordPress from admin,WordPress autosave the post with in every interval using AJAX call as a revision.Default interval set by wordpress is
60 seconds means in every one minute,wordpress saves your post and generate revision.
So,You can modify this interval using
Sometimes your browser crash or problem with internet connection or you forget to save content then the auto save interval feature can be used to restore your data back.
You can increase increase this setting for less numbers of revisions which is for longer delays in between autosave post.Also,You can decrease this setting as per your requirement to save content and not lose your important content
Add Following line of code into wp-config.php file.
define('AUTOSAVE_INTERVAL', 160 ); // Increase interval = Less revisions
define('AUTOSAVE_INTERVAL', 30 ); // Decrease interval = More revisions