Something retractable can be retracted: in other words, it can withdraw or collapse — like an umbrella or a cat's set of claws.
Cats have claws that can be drawn in to their paws or extended. Anything like that is retractable. Retractable things — like an umbrella — have a part that is concealed that can then be extended outward. A police baton is retractable if it can be short and compact or long and extended. A retractable roof can be extended as needed. Some sports stadiums have a retractable dome that can be put up when it's raining.