You have studied your magic and have become so attuned to it that you have learned to push yourself beyond your normal limits. As you cast a spell you can choose to add 1 or more spell slots to increase spell's level as if you were casting the spell at a higher spell slot (while casting a 2nd level spell and you can add a 1st level spell slot to equal a 3rd level spell slot). For each spell slot you added to the spell you gain 1 level of exhaustion after the spell ends.
You can only add them up to the highest spell slot you can cast. When using this ability you cannot concentrate on any other spell than the one being cast even if the spell being cast isn't a spell requiring concentration. You can only increase a spell's level. You cannot add spell slot levels together and then cast a higher level spell.