What protects us from cosmic background radiation?
Nothing "protects" us from cosmic background radiation. But it is of very low intensity and poses no danger at all.
Cosmic background radiation is essentially thermal radiation at a temperature corresoonding to the "background" of intergalactic space. But that is only about 2.7 Kelvins (it used to be hotter, but it has cooled off with the continued expansion of the Universe).
Most objects immediately around us are roughly 100 times hotter; according to the Stefan-Boltznann Law (https://en.m.wikipedia.org/wiki/Stefan%E2%80%93Boltzmann_law) this means the immediate ambient environment produces thermal radiation about 100 million times as intense as the cosmic background. It takes very sensitive instruments even to detect and measure the cosmic background radiation.