From Online Manual
- Optimize all tables to improve performance. Using this functionality causes SMF to instruct the database to attempt to reanalyze and reoptimize the tables in your database. It's not a good idea to do this too often, because it can take a long time.
- Check all files against current versions. This does exactly as it says and will tell you which of your files need to be upgraded.
- Find and repair any errors. Going to this page will search for any possible errors in your database. Usually, these errors are caused by improper conversions, modifications gone wrong, or data corruption. It's a good idea to check this once and a while, just in case.
- Recount all Forum totals and statistics. This recounts the number of posts and topics in each board, the number of personal messages each Member has, and a few other statistics. Use this feature if you suspect something has gone wrong.
- Empty out unimportant logs. This empties a few of the less important log tables. This isn't necessary unless you really want to save database space.
- Convert the database and data to UTF-8 This will convert your character set for both your database and data to UTF-8. This should be done with caution as it is not desirable in all cases. Upon clicking this link you will receive more information on the effects of converting to UTF-8. SMF will attempt to detect your character set for your data, but if you know your data character set is different from what is detected, you can change it using the dropdown next to Data character set.
This area allows you to save a copy of all the posts, settings, members, and other information in your forum to a very large file. For larger forums, this backup may not be able to backup your entire database, in which case you may want to backup your data by other means such as using phpmyadmin, or some other database backup utility.
It is recommended that you backup often, perhaps weekly, for safety and security.