の denotes ownership. Putting の after a noun makes it possessive. For example, わたし means I, or me. わたしの means my, or mine. By doing this, you can put a noun after の to make the first noun possessive of the second noun. That sounds a little confusing, but it's just like this:
A の B
A and B are nouns. A has ownership over B. わたしのねこ (my cat), ともだちのほん (friend's book), せんせいのけんきゅうしつ (teacher's office) are some examples. の can be thought of as "of" or apostrophe S ('s). So those translations can also be "cat of mine," "book of a friend's," "office of the teacher," and it would all generally still have the same meaning. English just has many ways to say one thing.
Hopefully this isn't all too difficult to understand. Start practicing this grammar! You will see it everywhere in Japanese writing and speech.