Optimizing Table in database is just like the doing Disk Defragmentation in your PC.
Optimize Table should be used if you have deleted a large part of a table or if you have made many changes to a table with variable length rows, such as VARCHAR, TEXT, BLOB or VARBINARY columns. Deleted rows are maintained in the linked list and insert operations reuse the old row positions. You can use “Optimize Table” to reclaim unused space and defragment the data file for optimal performance. If a lot of changes have been made to a table, optimizing the table can sometimes significantly improve performance.
If you try importing a mysql database backup file greater than 2 MB then you may receive an error – “No data was received to import. Either no file name was submitted, or the file size exceeded the maximum size permitted by your PHP configuration. See FAQ 1.16.”. No Worries! you just need to make few changes to your php.ini file.