Dvinsk has a direct rail connection to Velikie Luki, an objective of Army Group North. Army Group North's 4th Panzer Armee can have Dvinsk as an objective. The also reallocate divisions, air support points and ground support points between the Army Groups' reserves and allocate reinforcements. OKH has an additional responsibility of juggling forces between OKW in the west and forces in the east. Fewer forces in the west means a higher rate of attrition and a possible early end to the war.
High values are good.
Being on a high state of readiness and battles lower administrative and supply values. Units with low admin and supply values fight worse and are more vulnerable. The computer handles most of the complexity. In a player turn a player has some simple options. A Korps adjacent to an Army can Retreat, Defend, Probe or Attack.