Postcard
noun
A writing that you put on a leaf is like something else. Instead of a leaf, think about writing you put on a leafish sized item. A postcard perhaps?
The reading is kind of weird, sorry. The reading for 葉 is the は you know and love. Then, 書 is かき despite not having a き on the end like you'd expect. So, it's weird. It's shortened like a postcard is a shortened letter, I suppose? Technically it is the 書き reading, but keep in mind that 書き gets rendaku'd.
ポストに葉書を出してきてくれない？
Can you go put a postcard in the mailbox for me?
その葉書には手書きのメッセージが書かれていました。
There was a handwritten message on the postcard.
彼女は毎回、旅行先から葉書を送ってくれます。
She always sends a postcard from her trips.