In plain words
Out of joy and zest they have enjoyed splendours such as these.
There is another meaning to this too, a scientific one. The season just gone by is the rainy season. Wherever you look there is water and mud, and through filth of this kind insects and vermin of many kinds go on multiplying. Through such insects and vermin, ill health spreads among men. By burning sparklers of this kind, by this smoke, by these poisonous powders, those insects and vermin are destroyed. Through this it makes man's health and happiness grow.
It is on many kinds of inner meaning that the festivals of the Bhāratīyas have grown up. Here is one more thing. In keeping with the Vedic saying Ekoham bahusyām, here today we light lamps. In many ways we make this decoration. But first of all we light just one lamp, and through that lamp we go on lighting all the lamps. What lights these many thousands of lamps is one lamp. That one lamp alone has gone on lighting the many lamps. That one lamp is the Supreme Light. The rest are the lamps of life. This Supreme Light alone, the one light, has gone on lighting all the lamps of life: that is Ekoham bahusyām. That one lamp itself took form as the many lamps. This day of Dīpāvali is the day that showed forth that spiritual meaning too. But all these lamps drive away only the outer darkness. This light has one more virtue as well. Wherever a lamp is placed, it travels only upward, never downward. That is, it goes only to a higher level; it journeys only towards the path of Jñāna, the path of merit.
To light a lamp, four things must be there. One, a clay lamp. Second, oil. Third, a wick. Fourth, a matchbox. If any one of these four is missing, we cannot light the lamp. All four must be there without fail. But what darkness does all this drive away? It drives away the outer darkness. Once, Hanumān, on his search for Sītā, set the whole of Laṅkā on fire, sat on top of a hill and looked out over all Laṅkā. That day the whole city of Laṅkā, which had been wrapped in pitch darkness, shone as bright as broad daylight with this fire. Rāvaṇa was mighty. He was an adept who had mastered the sixty-four arts. He had a son named Meghanāda, of great might and prowess. He had the mighty Kumbhakarṇa as his younger brother. He was the younger brother of the immensely wealthy Kubera. He had ten heads and twenty arms. Yet, for all he had, could anyone save him? Though the whole city of Laṅkā shone as bright as broad daylight, within Rāvaṇa's mind a pitch dark called darkness was closing in: Ajñāna.