Key Lime is Skin in surviv.io that was added in the v.2.0 update on January 17, 2018. It is a neon yellow skin with bright yellow hands and an orange-brown backpack.
It is only cosmetic, there is no practical value to it. In fact, it is more likely to alert other players of you because of it's bright coulor.
The clothing article is named after Key Lime, a kind of fruit that shares the same color.