By the 1500s, galleons, huge sailing ships, were taking cargo across oceans all over the world.
The second sentence defines a galleon. To combine the two sentences, an appositive phrase is necessary in order to insert the definition of galleon without creating a run-on. Appositive phrases are used to define or clarify a noun. They are typically surrounded by commas. Often an appositive phrase can be erased without damaging the meaning or grammatical structure of the sentence. Another example of an appositive phrase is: Jim, my brother, is happy. The appositive phrase "my brother" explains who Jim is.
