The phrase "invisible shield" refers to something that cannot be seen but offers protection or acts as a barrier to keep things safe. It's like having an unseen force field that keeps things away or prevents harm. Full definition
Situated atop a bedrock of all - powerful vibranium, Wakanda has developed technology that positions it far ahead of the rest of Earth — flying cars and everything — but to protect its people from colonial predations, the country's rulers have cloaked their cities and marvels behind invisible shields, the better to pose as «just another» Third World nation. (bostonglobe.com)
Rather than actually appearing during dungeon gameplay, your Vassals serve as invisible shields for your Lord, taking damage that would have been dealt to him during combat. (esperino.com)
has a line of protectors that work to create an invisible shield over shoes allowing them to weather the storm and stay protected against water and stains throughout the winter months. (thefashionablehousewife.com)