Yolochka is a Christmas tree, that's right.
This phrase is our tradition.
When the New Year celebaration begins, but the garlands of multicoloured electrical lights on the Xmas tree are not lit yet... Well, first the children call Ded Moroz (our Santa Klaus). They should say "Dedushka Moroz" three times, loudly, all together. Then Ded Moroz comes and they call his granddaughter Snegurochka. She comes and then they say three times "Yolochka zajgis'!" which means "Xmas tree, light up!" and the multicoloured lights turn on. Children like this tradition very much.