How are dolphins able to jump out of water?

For the same reason that people can jump up and down and planes can fly: power. Gravity is not of infinite strength. If it were, we wouldn't be able to do simple things, like raise our feet to walk or lift a book to read. Gravity is powerful enough to ensure that things don't float around, but it's not strong enough to prevent them from leaving the surfaces on which they rest if an adequate force is applied to them. Only a small amount of force or power is required to overcome the force attributable to gravity -- also known as weight. A swimming dolphin can easily produce enough power to generate enough speed to propel it through the surface of the water and continue through the air for a few meters. Thankfully, gravity is strong enough to ensure that what goes up must come down. Once the dolphin can no longer propel itself, it falls back into the ocean.