Porting your wastegate allows increased flow rates through the gate itself.
The gate is a little metal flap (operated by a vacuum operated actuator) that covers a small hole in the rear of the turbo housing hot-side (exhaust side).. The flap usually extends past the diameter of the gate port/hole so that no air can leak. When the flap opens, it allows boost pressure to escape, and the turbo doesn't over-boost.
Porting the wastegate entails using a dremel (or similar) to increase the diameter of that port. Usually, you mill enough material out so that the stock flapper still can cover the hole completely. Sometimes, people upgrade that also when going larger (although at this stage you'd be looking at an external gate - this means that the internal gate hole is sealed shut, and a separate wastegate is attached to the exhaust manifold).
Generally, you'd be wanting to port the gate out when you're experiencing 'wastegate creep'. This means that the exhaust flow is exceeding the flow capacity of the wastegate, allowing for an uncontrolled rise in boost pressure.
Usually, you won't experience this on a stock turbo.