Complex sentence: When they do react with the quarks within the ice, they can create charged leptons (electrons, muons, or taus) which, if they are energetic enough, can emit Cherenkov radiation.
Split into simpler sentences: When they do react with the atoms of ice, they can create charged leptons (electrons, muons, or tauons). These charge lepton can, if they are energetic enough, emit Cherenkov radiation.