Congratulatory Gift, Congratulatory Money
noun
To celebrate a ceremony, you offer a congratulatory gift. 祝儀 generally refers to monetary gifts for celebrations, such as congratulatory money for weddings. It can also include money given away when you make a hole-in-one in golf or win big on a gamble. While 祝儀 can also refer to the celebration itself, this use is not as common.
しゅうぎ
This is a jukugo word that uses the on'yomi readings of the kanji. You just need to remember that 祝 is read as しゅう for this one and you'll be fine.
封筒の表には「御祝儀」と自分の名前を、裏には金額を書いたよ。
I wrote "congratulations" and my name on the front of the envelope and the amount of money on the back.