When it's pronounced, it sounds more like pescalo, why is this?
Generally a spanish 'd' is very soft; borderline 'th' sound, and some latin american accents almost completely leave the 'd' out sometimes. Almost like a 'pescao'. Duolingo's voice is just using this very soft 'd' sound. When pronouncing the spanish 'd' you should have your tongue almost poking through your teeth.
The computer voice gets glitchy sometimes. "D" always sounds like "d" in Spanish. Other than that, "péscalo" literally means "Fish it!" (or, figuratively, "Catch him (in the act)", or "Watch him," in the sense of "look at what he's doing").
pescalo isn't it's own word. There is a verb the comes from the word "pescado" (fish) that is "pescar" (to fish). When you put it into the imperative tense (using the el,ella,usted form as a command) you can add the pronoun "lo" (it) at the end to mean "do this verb (fish, in this case) to this thing (it, since we don't know what lo refers to in this case)"