1 m³ is literally a cube with sides that are 1 m in length. 1 m = 100 cm, so each side of this cube is 100cm in length. Finding the volume means multiplying length by width by height: 100(100)(100)=1,000,000 cm³ in 1m³.
We know that we have 25,000cm³ of mulch; we divide to find how many cubic meters this is: 1,000,000/25,000 = 40 m³.