You have learned how to cast rituals spells that are written in a ritual book, which you must have in hand while casting one of them.
When you choose this feat, you acquire a ritual book holding one spell from a theme you know with a mana cost of 1 and the ritual tag.
If you come across a spell in written form, such as a witch’s grimoire or a mage’s spellbook, you can add it to your ritual book if you can decipher the unique system of notation used to write it, it costs equal to (or less than) your mana limit, and if you can spare the time and costs to copy it. For each mana cost of the spell, the process consumes 5 mana, takes at least 5 hours, and costs 100 sp. The cost represents materials you expend as you experiment with the spell to master it, as well as the fine inks you need to record it.