Mr.
Ms., Lord
suffix
The kanji means Milord, and this word used to be a name ender similar to the English title Lord, but nowadays it's closer to Mr. or Ms.. It's rarely used in speaking or even written correspondence these days, but is still used in official documents, and you'll probably see it most on certificates. If in doubt, you're better off choosing the name ender 様 because it's much more widely used, and also more polite than 殿.
Since this word is made up of a single kanji, it probably uses the kun'yomi reading. You did not learn this reading, so here's a mnemonic to help you to remember:
Do you know a Lord? Imagine it being someone named Mr. or Ms. Don (どの). If you know a Don, that's great, imagine that person. If you don't, think of a famous Don that's out there.
この賞状は、中村コウイチ殿の優れた功績を称えて与られます。
This certificate is awarded to Mr. Nakamura Koichi in recognition of his outstanding achievement.
鈴木殿にお伝えください。
Please tell Lord Suzuki.
名前の後に「殿」を付ける用法は、現代では同列か目下の人に対しての敬称として、書面にのみ用いられる。
The use of "dono" after a name is used today only in written documents as an honorific title for those who are equal or inferior to you.