In the initial stages of learning to read, a reader is so focused on decoding the words on the page, that he doesn't have much mental energy left to spend on meaning. To decode the words, a beginning reader sounds out the words - he is connecting sounds to the letters he sees and trying to blend those sounds together to form words. Then he must know what the word means.
If a reader encounters an unfamiliar word, the decoding is much more difficult because he then has to try to get the meaning of the word from the context, from the surrounding words. That, however, means that the reader must be able to decode those surrounding words - and remember them, and then figure out the meaning of the unfamiliar word.
As a reader becomes better at decoding the words, he'll be able to read words more quickly. But that does not mean that he'll be able to read with expression. Reading with expression means that a child is not reading in a monotone with all words getting equal emphasis. Knowing which words to emphasize requires that a reader understand the meaning, not just of the individual words, but of entire sentences. He must also understand the significance of the words and sentences. That means that if he is reading a story, he must understand the story. Notice the difference between these two readings from the The Three Little Pigs: "I'll. Huff. And. I'll. Puff. And. I'll. Blow. Your. House. Down." "I'll huff! And I'll puff! And I'll BLOW your house down!"
The second is a fluent reading; the first is not.