Complex sentence: A gas-discharge lamp is an example of a device having "negative" resistance, where after lamp ignition, the increasing lamp current tends to reduce the voltage "dropped" across it, (if it is in series with other circuit elements).
Split into simpler sentences: A gas-discharge lamp is an example of a device which, under certain conditions, has "negative differential" resistance. In such a situation (after lamp ignition), every little increase in the lamp current tends to reduce the voltage "dropped" across it (supposing the lamp to be connected in series with other circuit elements).