Omeka uses a standard of metadata called Dublin Core to describe items. Using standardised metadata is useful as it makes it easier to cross-reference your collection with other collections or migrate your collection to a different platform.
Omeka provides for the addition of extra metadata fields and the creation of custom metadata. For example a Person type has fields for dates of birth, death and occupation. There are prompts built in to Omeka that will help you understand what information to put in every Dublin Core field. There is an explanation about what information each of these fields is intended to contain in our Dublin Core Metadata Knowledge Base article.
It's a good idea to start planning your Omeka site before you start building it.