The electronic configuration of Co2+ is 3d7. In an octahedral complex of Co2+ with H2O ligands, the d orbitals split into two sets of energy levels: t2g (lower energy) and eg (higher energy).
In this case, since the complex is octahedral, we have six ligands surrounding the central cobalt ion. Each ligand donates a pair of electrons to the cobalt ion, occupying the available d orbitals.
So, [Co(H2O)6]2+=d7(Highspin)=t2g5eg2
In the t2gset of orbitals, there will be 1 unpaired electron