XBRL uses dictionaries, known as taxonomies, to group financial concepts, or elements, together. A taxonomy defines a tag for an item of business data, its properties and its relationships to other elements. For example, if we are working with a balance sheet item such as Cash, the taxonomy would include the definition of Cash and define how Cash relates to other balance sheet items in the taxonomy (e.g. Current Assets and Total Assets).
Taxonomies are organised in hierarchies with parent and child relationships between tags. Parent tags are more general and child tags are more specific. For example, Profit (loss) is the parent to child tags like Revenue, Cost of Sales, Gross Profit etc. You can see this represented visually in the Tagger - child tags are indented under their parent tags:
Since different countries report in different ways, there are many taxonomies available with definitions to reflect local accounting and other reporting regulations. For example, the International Financial Reporting Standard (IFRS) taxonomy is different to the US Generally Accepted Accounting Principles (US GAAP) taxonomy.
Remember that XBRL stands for eXtensible Business Reporting Language. This means that it can be added to (i.e. extended) if it does not have the correct concepts for what you are trying to report. When you create your final report you will automatically create a custom version of the taxonomy which includes the tags you have used and any extensions you have created.