By default when a user browses a category they will see assets that have been explicitly put into that category.
This behaviour can be changed so that they see assets in that category but also assets that are in any categories that are a descendant of that category. I.e. you see everything at the top level and navigating into categories effectively filters the assets into smaller more specific sets.
For example, suppose that you have a category called 'Animals' that has two sub-categories called 'Cats' and 'Dogs'.
If an asset is placed in the 'Animals' category then a user browsing the top-level 'Animals' category will see that asset. However, by default, they will not see any assets placed in the 'Cats' or 'Dogs' categories.
To change this behaviour go to Admin > Attributes > Display Attributes > Browse then open "Update browse settings" and change the following setting to true:
Then, a user browsing the 'Animals' category would see assets that are in 'Animals', 'Cats' and 'Dogs'.
It can be a good idea to do this if you want to make the top-level category pages more visual (i.e. as they will then show images) or if you don't have a large number of assets in each category.
If you have a very large number of assets, or if you want to explicitly place certain assets in the parent categories, then you should leave that setting to false.
Note: there is an equivalent setting for access levels:
If you are on an older version of the software, then the configuration options may not be available via the front end as shown above. If so you will need to either upgrade or use the following settings in the settings file: