I'm assuming that you already know how to add new keywords in the keywording box and that there is also a keyword list, which contains all the keywords you've used in your catalogue. This video on Adobe's website should bring you up to speed if you're not sure what I'm talking about. The hierarchy allows you to place some keywords as children of other keywords. Whenever you add a keyword that is lower down the hierarchy, Lightroom will automatically apply all keywords up the chain. For example, if you do a tour of Scotland, you might have keywords for places such as Edinburgh and Loch Ness, and you can make these children of the keyword 'Scotland'.

Placing a keyword beneath another in the hierarchy is as simple as dragging and dropping it in the keyword list. New keywords can also be created beneath existing keywords by right-clicking on the keyword and selecting 'create keyword tag inside…' in the drop-down menu.

Once you've placed a few keywords into a hierarchy then this saves you needing to manually add the higher up keywords, for example you won't need to add the keyword 'Scotland' to your photos of Edinburgh. You can add as many levels as you want to your hierarchy.

Other keywording tricks

As well as children and parents, keywords can also have synonyms. These are keywords which are automatically applied in the same way as parents, but which do not have to be in the hierarchy. For example if you have a keyword which should automatically be applied alongside a number of other keywords in different parts of the hierarchy then you can have it as a synonym of these keywords.

There is an option to have keywords which are part of your hierarchy but which will not export with the photograph. Situations where this feature is useful include those where you need to differentiate between two identically named keywords (as each keyword must have a unique name) such as Boston (the town in Lincolnshire) and Boston (the American city). You can call the two keywords something like 'Boston, UK' and 'Boston, US', add 'Boston' to the synonyms for both and then set the keyword to not export. That way whichever Boston you add as a keyword, when you export the photo it will just have the keyword 'Boston'. Another situation might be if you want to group some keywords together in the hierarchy but you don't want the name of the keyword you have grouped them under to export.

Both these last features can be accessed by right-clicking on a keyword and selecting 'edit keyword tag'.

Using these advanced features will mean that the list of keywords that are applied when you export isn't the same as the list of keywords you have applied to the photo. It will include parents and synonyms and exclude keywords set to not export. If you want to view the list of keywords for any photo, there is a drop-down menu above the box where you type keywords in. Setting this to 'will export' will show you those keywords.

This is just one way of using Lightroom to supercharge how you organise your photographs, and metadata is increasingly important as a way of both sorting your images and commercially, a way of helping others to find them so it's definitely worth the investment in time to learn the tricks. This link will take you to a more detailed, and much better written description of the keywording features.