The electron configuration for manganese is 1s2 2s2 2p6 3s2 3p6 4s2 3d5. It can be shortened to [Ar] 4s2 3d5, where the [Ar] represents argon, the last element in the third row of the periodic table, whose electrons fill every shell prior to the 4s-orbital.
The first number in each grouping represents the energy level. The second number represents the type of orbital. In addition to the s-, p- and d-orbitals, electrons may also be in f-orbitals. The final number in each grouping represents the total number of electrons in each orbital. The only orbital that is not full is the 3d-orbital.