Complex sentence: The discharge is terminated by the collective effect of the positive ions created by avalanches, which have lower mobility than the free electrons and remain in the area of the anode wire and counteract the electric field which is necessary for continued avalanche generation.
Split into simpler sentences: The discharge is terminated by the collective effect of the positive ions created by the avalanches. This causes a "space charge" which counteracts the electric field which is necessary for continued avalanche generation.