Trademarks are often categorized on a "distinctiveness" scale from generic, descriptive, suggestive, arbitrary, and to fanciful.
Generic trademarks have been difficult, if not impossible, to register. At the other end of the spectrum, fanciful trademarks have been the easiest to register.
A fanciful trademark might be "Kodak" film. An arbitrary trademark might be "Camel" cigarettes. A suggestive trademark might be "Tide" detergent. These types of trademarks have a higher probability of registration because they are "inherently distinctive".
A descriptive trademark might be "Park 'N Fly" airport parking. This type of trademark is more difficult to register because it is not "inherently distinctive".
A generic trademark is the name of the good itself - such as "Wine". Generic trademarks do not distinguish one person's goods (e.g., wine) from another's goods (e.g., wine)
Now, it may be easier to register a generic term, if one is willing to add a domain extension - like .com or .org.
The US Supreme Court concluded that when the trademark is viewed as a whole, rather than its individual parts, BOOKING.COM was not generic. The Court, as an example, explained that a travel consumer would not ask another person for their favorite "Booking.com" reservation provider. And a travel consumer would not understand Travelocity to be a "Booking.com".
There did not appear to be disagreement that the word "Booking" alone was generic. It was the addition of .com that made "Booking" non-generic.