That's probably a memory configuration problem in your server's MySQL setup.
You'll need to report the error to your hosting company to have them resolve it.

You could try using phpMyAdmin and selected your "categories" table, and then clicking on the "Operations" tab and click on "Optimize table". Then do again for "categories_description" table.
This may or may not improve things somewhat.
You could optionally also click on "Repair table" in the same manner.
If these don't resolve your problem, your hosting company will have to get involved to fix their server configuration problem (probably a lack of sufficient memory).