In cyclopentane since the angles are about 108o there is no significant destabilization due to angle strain. There are three extremes in the conformations of cyclopentane.
Planar - The planar conformation has significant angle strain since all 5 hydrogens above and below the ring would be eclipsed. This would be the least stable conformation
Envelope - More stable than planar cyclopentane and about the same stability as the half-chair conformation. Notice that four of the carbons are coplanar resulting in significant torsional strain between eclipsed hydrogens.
Half-chair - More stable than the planar geometry since some of the torsional strain is relieved. In this conformation three carbons are coplanar, while two are out of the plane.