ok so can someone explain this to me? what does it do?

Soultrap is a mysticism spell that you cast on enemies. If you kill the enemy before it wears off, you trap their soul into a soul gem. Soul gems can usually be found at mage guildhalls and the arcane university. You must have a soul gem equal or greater to the type of soul your trapping (i.e grand soul gems usually go to Demons and the like).

Soul gems can then be used to enchant your gear. If you have access to the Arcane University (you get this after you complete all the recommendations for the mages guild) then you can go to the chriosanium and use the altars there to enchant. The interface for that is generally self explanatory.

You can also soultrap your own summoned monsters and steal their souls.
Just wait for its time to almost run out, whap a soultrap on it and sit back.