what are sigma electrons and what are pi bonds? please explain in detail.

Sigma electrons are the electrons which form bonds that lie on the internuclear axis of the molecule. It is the bond that is formed on the line joining the 2 nuclie involved in the bond formation. These are the stongest bonds. The orbitals indulging lie directly on the axis.

Pi bonds are formed by the p orbitals which lie perpendicular to the line joining the 2 atoms or the inter nuclear axis. These bonds are weaker in terms of strength and are secondary in terms of preference.
A double bond between 2 atoms will have 1 sigma and 1 pi bond.
A triple bond between 2 atoms will have 1 sigma and 2 pi bond.