"I light them."
Translation:Je les allume.
In French, the direct object pronouns me, te, le, la, nous, vous and les go before the conjugated verb.
I recommend that you use the lesson notes on the desktop version of Duolingo as they are very helpful in explaining concepts like this. Here is another link with more information on pronouns: https://www.frenchtoday.com/blog/french-grammar/understanding-french-pronouns
Allumer is a regular -er verb, and is conjugated as such: j'allume / tu allumes / il/elle/on allume / nous allumons / vous allumez / ils allument
And even if the direct object (les in this case) is plural, it doesn't change the conjugation.
I think that would mean, "I illuminate (shine a light on) them." Je les allume means rather, "I light them." As in, lighting candles or even turning on light fixtures.