a shield planet is a planet that has been completely shielded by placing a massive shield generator at the center of the planet. There were only three planets in the entire UNSC that were shielded; Earth, Jericho, and Reach.
Cost The cost of shielding a planet was tremendous, the UNSC could easily build ten massive fleets for the cost of just one of these shields.
How it works The technology was tricky and if performed wrong could destroy the planet. the shield generator was first developed in 2561 a couple of years after admiral Hood had obtained the shielding technology from the elites but there was no power supply that would sustain the generator, well not unless you had a black hole handy. The power requirements were tremendous but scientist were able to develop a system were the generator was powered by the core of the planet. The design was developed on reach and when the UNSC first wanted to equip it to earth they had to spend another year doing research before they could properly get the generator to run on Earth
Planets that are shielded In 2565 Earth received its generator and in 2567 so did Reach. Jericho didn't receive its shield until three years after Reach because of its size and even then the shield around Jericho was still seventy five percent weaker than Earth's or Reach's but never the less, in 2568 Jericho received its shield upgrade making it the third shield planet in the UNSC.
If you watch the Halo 4 Teaser Trailer, you will see a shield planet (unknown) to which the Chief is drwaing towards. It does, however, appear to have some forerunner structure.