Joomla stores its session metadata in the #__session database table — as long as the “Track Session Metadata” option is enabled in your site's Global Configuration.
By the nature of this feature, the #__session database table gets a high volume of read / write operations. For reasons that have to do with how MySQL works this can cause the table to have a lot of wasted space, making it slower, or even stop accepting new writes. In the former case page loads will take longer to complete, even for Guest (not logged in) visitors to your site. In the latter case you may find it impossible to log into your site or even that some or all pages of your site no longer work.
It is possible to avoid this situation by asking MySQL to optimise this database table, removing wasted space. Moreover, if the table appears to be broken we can ask MySQL to attempt to repair it. This is what this task type does.
We recommend running this feature once an hour on very busy sites and once a day on sites which receive a lower volume of traffic.