The quark-gluon plasma is a hot dense soup of (mostly) free quarks and gluons. It is created in heavy ion experiments like ALICE, and ATLAS Heavy Ion at the LHC or STAR at RHIC, among others, via two large (typically gold or lead) nuclei smashing together at close to the speed of light. However, this "free" quark state is very short lived, and it almost immediately cools down and re-hadronizes into confined states which are measured in the detectors for these experiments. So, we don't actually "see" free quarks but we can deduce that they must have been. In nature, the only place you might be able to find deconfined quarks is at the center of a neutron star, but that hasn't been observed.