Leaky things accidentally allow water (or gas) to pass through them. A leaky bucket is a bad thing, and a leaky diaper is even worse.
A leaky hose lets water squirt out at different points, and a leaky helium balloon plummets to the ground, deflating as it falls. You can also talk about a leaky government, or a leaky newsroom — in other words, a group or a person guilty of exposing information that's meant to be kept secret or confidential. This kind of leak is a "deliberate disclosure of information," and it dates from the 1950s.