One pie acceptor here
Anyway, jokes aside:
Those are classes of ligands in coordination chemistry.
Ligands can be classified as:sigma donors
- when coordinated to metal, it forms one single sigma bond,for example NH3sigma donor and pi donor
- when coordinated it forms one sigma bond and one or more pi bonds. The important thing here is that he pi bond is formed by moving electrons from ligand to orbitals of metalsigma donor and pi acceptor
- the sigma bond is formed as in examples before, but there is donation of electrons from metal to empty pi-anibonding orbital
There are some other types - for example sigma dnnor and d-acceptor where the back donation is to empty atomic d-orbitals of atom coordinated to metal.
Hope it helps, I tried as simple as possible but to answer this question some knowledge from general and inorganic chemistry is required