Complex sentence: The second part, "Zelderheide" means: moorland of Zelder, concluding the town consist of an area that used to be peat, an area with moorland and an area called Zelder.
Split into simpler sentences: The second part, "Zelderheide" means: moorland of Zelder. At the time the town consisted of three areas: an area of peat, an area with moorland and an area called Zelder.