Complex sentence: The "ribbon" or presynaptic dense body is a unique structure at the synapse's active zone, hovering several nanometers above the presynaptic membrane and tethering 100 or more synaptic vesicles.
Split into simpler sentences: The "ribbon" or presynaptic dense body is a unique structure at the synapse's active zone. It hovers several nanometers above the presynaptic membrane and tethers 100 or more synaptic vesicles.