A Magento site contains a large amount of data relating to the products and categories on the site. To speed up the process of accessing that data, a reindexing of the database tables is required when changes are made to the site.
You can access the Index Management page by logging into the back end of Magento and selecting System>Index Management.
The Index Management screen lists the indexes that are available to reindex, a description of each index, the current update mode selected for each index, the current status of each index, whether an update is required for each index and the date a reindex was last run on each index.
Reindexing Your Data
In most cases the Update Mode will be set to Update On Save but there may be occasions when you will still need to reindex to see recent changes appear in the front end of your site. The Status column of indexes requiring an update will say Update Required and be coloured red.
You just need to click the reindex data link next to the index or to reindex multiple indexes, tick the boxes next to each index that requires it, select Reindex Data from the drop down in the top right and click submit.
Changing the Index Mode
By default, all indexes are set to update on save. This means that when you make changes in the back end and click save, the reindexing is done automatically.
This can be set to manual by using the tick boxes on the left to select an index, selecting Change Index Mode from the Actions menu, then selecting Manual Update from the Index Mode menu and clicking submit.