Answer:
If you can prevent a nonpaying consumer from having access to a good or service, that good is excludable. For example, only clients that buy a ticket can enter a movie theater and watch the movie.
A good or service that can be consumed by only one consumer (or a specific number of consumers) is rival in consumption because other people cannot consume them simultaneously. For example, once the movie theater sold out, no more people can get inside and watch the movie.
