Plain English Meaning
TSO C-115a is an FAA rule that says what a certain class of GPS-based navigation box has to be able to do before it can be installed in an aircraft. Equipment built to this rule is allowed to help with navigation, but the pilot still has to back it up with other navigation equipment.
Full Definition
TSO C-115a is an FAA Technical Standard Order that sets the minimum performance standards for airborne area navigation equipment that uses multi-sensor inputs, including GPS receivers integrated with other navigation sources such as VOR, DME, or inertial systems. Equipment certified to TSO C-115a is approved for supplemental navigation use only and is not authorized as a stand-alone means of navigation under ; it must be used alongside other approved navigation equipment.