what are p pi-p pi bonds?

The $p\pi - p\pi$ bonding you can see in the CO molecule.

 In CO molecule both carbon and oxygen atoms are sp hybridized. one of the sp hybridized orbital of each atom is used for the formation of C-O sigma bond. While other sp hybridized orbital contain lone pairs of electrons. Two unhybridized 2p orbitals of each atom constitute $p\pi -p\pi$  bond as shown.

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