You have learned to regain some of your magical energy by studying your spellbook.
Once per day when you finish a short rest, you can choose expended spell slots to recover. The spell slots can have a combined level that is equal to or less than half your wizard level (rounded up), and none of the slots can be 6th level or higher.
For example, if you’re a 4th-level wizard, you can recover up to two levels worth of spell slots. You can recover either a 2nd-level spell slot or two 1st-level spell slots.