If you're able to juggle, you'll go far in life. To be able is to have the skill, knowledge, or permission to do something.
Able is an adjective that describes the ability to do a particular thing, like your friend who's able to ride a motorcycle and your dog that's able to roll over on command. It can also mean very skilled or capable, as when you describe your sister as an able tap dancer. Able comes from the Latin word habilis, "easily handled or apt." The h is silent in Latin, which led to it being dropped from the English able.