Something that “impinges” a nerve is something that---usually negatively---affects the nerve. Common results of nerve impingement are numbness, tingling, pain, hyperesthesia (excessive, or pronounced, sensation), or hypesthesia (diminished sensation).
Often nerve impingement is conceptualized as pressure on a nerve, but there are also other kinds of mechanical impingement. Abrasion, stretching, twisting are some examples. Temperature changes or chemical impingement are also possible.
In my practice, the goal of relieving pain usually involves relieving mechanical nerve impingement, accomplished by careful, gentle, precise adjustments with the Activator instrument.