A subdomain is a subdivision of a domain. There has to be a registered domain before you can have a subdomain.
You can then assign several subdomains (the number of subdomains available depends on the package you are using) depending on your needs. For example, websites can be divided up systematically using subdomains. They are often used to highlight certain services or products and make them more accessible. You can also assign email accounts to each individual subdomain - e.g. for different departments or branches.
ionos.com is a domain, while the subdomain login.ionos.com refers directly to the IONOS login page.
All the letters from A-Z, digits from 0-9, and hyphens (-) are allowed for a subdomain name (as long as 2 or more hyphens are not consecutive). The name must start and end with a letter or a number, and spaces are not allowed.
Other than this, you can freely assign any name for a subdomain. For example, you may want to distinguish between different parts of your company with subdomains, such as "sales.example.com", "mobile.example.com", or "nyc.example.com".
Using the example above, you could then create email addresses using the subdomains, such as "firstname.lastname@example.org" or redirect the subdomains to a specific page of your website or another website entirely.