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.