In Mein Panzer, with some exceptions, a "unit" is a platoon, troop, or battery. A stand represents a section/squad, gun, officer, or vehicle. Each nations military has its own way of organizing its forces, but generally a platoon is 1 officer, and 30-40 men, a battery is 1-3 guns, and a troop is 2-4 armoured vehicles or 3 squads of combat engineers. In game terms, a platoon would have 1 officer stand, 3 - 4 squad stands, 1 - 3 weapon stands (mortars, machine guns, anti tank weapons). A battery would have 1 - 3 gun stands. A troop would have 2 - 4 vehicle stands, or 3 squad stands of combat engineers.
When you activate a "unit" you activate the entire unit. So if you activated a platoon, you would activate the officer stand, all the squad stands, and all the weapon stands at the same time.
To understand how army formations work in Mein Panzer, you do require a basic knowledge of real world military organization. The idea of a unit will make a lot more sense when you understand how everything fits from section/squad - platoon/battery/troop - company - battalion/squadron - regiment - brigade - division - corps - army - army group.
This video should help explain things a little better.