Encumbrance is a derived Attribute from the value of your Strength Attribute x2.
It determines, how many Items of what Size you can carry about your person or in your knapsack. All Items have a Size property, starting from zero.
The value for Size of your Items will add up, with your Encumbrance going red when the added values go over your Strength x2, which will result in Instructors reprimanding you for bungling about overencumbered.
You would then have to either dump Items in your Wardrobe, sell them, or raise your Strentgh in some manner.
Size/Encumbrance and Concealment/Concealbility are unrelated safe for the negative effects of being discovered by an Professor.
There are Spells to reduce the Size of Items.