The domain name is a component of a uniform resource locator (URL) used to access web sites, for example: URL: http://www.example.net/index.html. Top-level domain: net. Second-level domain: example.
A domain name allow you to have services like a website, emails or a game server and not have to type the IP every time, but instead use your domain name.
Once you order your domain name you can either connected to a ThunderHosting Service or access the free DNS management panel and point an external service or server.
If the domain you are looking for is not available it means that someone else as it already.
You can try searching for the same name with another TDL such as .your-country instead of .com
You should remember that your domain is the image of your project.
Sow you should chose a domain that represents your name, project name or what your do.