Shield Projects emit an energy shield in an arc, a wall that only needs energy to sustain it. The more damage a shield segment takes, the more energy it will draw.
Shields can be disabled with a hotkey, the default key is "R".
Shield deactivation can be automated by putting shields on an electric network an cutting the power with a power switch. The power switch can be controlled with a signal from a gate, for example.
Shields block most entities from moving through. You can get through or over shields with the Grappling Gun mod or the Jetpack mod.
For ground defence, shields are designed to be used in high priority, high threat areas, like a choke point where you know heavy attacks will hit your defences. They are not designed to be used in extremely high numbers along long continuous walls. If you have thousands of shield projectors it will reduce your game's performance.
If used in Space Exploration, shield projectors are a good late game option to protect spaceships in conjunction with lasers.