The domain name system has a hierarchical structure:

The component furthest right to the domain name is called the top level domain.  Top level domains are often country codes, for example .be (Belgium), .nl (Netherlands), ....  However there are also generic top level domains, for example .COM, .NET and .ORG

The second level domain is the component chosen by you, on the left of the top level domain.  So tuonome.be is a second level domain.