Etymology and English translation based on Anvaya (logical connection of words) of Maharshi Dayanad Saraswati (M.D.S)- -

(yaḥ)=That, (pavitraḥ)=Holy (ca) =and, (upacitaḥ)=plentiful, (vidvān) =scholar or (bhautikaḥ)=physical, (agniḥ)=fire, (asti) =is,(ca)=and, (saḥ)=that, (ayam)=this, (nimnam)= to the downward place,(gacchati)=goes, (iva)=in the same way, (śucīnām)=between the pure and the sanctified (śatam) countless, (śataguṇaḥ)=hundred-fold, (vā)=in othe words,(samāśirām) =Those who serve with virtue in all respects and from all directions, (sahasram)=to many (vai)=definitely (dvā)=two, (ādhārabhūtaḥ)= basic causatives, (dāhakaḥ) dāhakoṃ ke=as flammables (rīyate)=knows.

English Translation (K.K.V.) -

One who is pure and rich scholar or material fire and that goes to this lower place. In the midst of such purifying and sanctifying ones, one surely knows the countless, hundred-fold or many who serve with virtues in all respects and from all directions, as the two basic causatives.

TranslaTranslation of gist of the mantra by Maharshi Dayanandtion of gist of the mantra by Maharshi Dayanand -

There is simile as a figurative in this mantra. It is the fire, the famous form of the Sun and electricity that purifies and cooks hundreds of the substances. Just as water flows down to the places, similarly this fire going up in other words, setting fire down and water up, the combination of both produces velocity et cetera.