Shield, is an item you can get in the game Running Fred. It is also counted as a one-time use only skill.
Shield can be used to protect the player from most hazards that are lethal and can injure Fred, such as spikes, pendulums, pins, flames, explosives, etc......but not falling damage and swipes from Grimmy. So, if you fall into an abyss, or a pool of fire, this won't come and save you. Same for the deadly Grim Ripper. You will have to use the Safety Spring instead for falling, and use the Afterburner for the latter case.
Back to the topic. The Shield activates automatically, so you don't have to be scared that your reflexes aren't great enough. This proves to be useful as it spares you from most damages at the first time. However, every item has a downside (except for some, I guess). Shields only activate once per life, so you won't be able to walk through a level without moving while buying for about 1000 Shields.
A Shield looks like a large white shield, lined with some screws around it. It is extremely durable, able to reflect damage ranges from flames, perma-frost tubes to large swinging pendulums and sharp spikes. However, it is still not durable enough to let you recover from a fall to the lava pool and abyss.