[Heisenberg97]

1) What actually is a weak field/strong field ligand and how does it lead to high and low spin respectively?

2) Why don't transition metal complexes exist when the transition metal has a high oxidation state? (e.g. FeO4 with Fe in the +8 oxidation state)

[AWK]

There is no such compound as FeO₄. FeO₄^2- salts exist. relatively stable OsO₄ is known.
For 1)
read wikipedia and a few times your textbook.
https://www.wikiwand.com/en/Ligand_field_theory
http://chemwiki.ucdavis.edu/Core/Inorganic_Chemistry/Crystal_Field_Theory/High_Spin_and_Low_Spin_Complexes