The vertical lines you see on the music staff are called bar lines. Their job is to separate a line of music into sections or measures. For instance, the bar lines in the middle of this example break up an otherwise long line of music into four different parts.
Chunking Music
Bar lines organize groups of notes and rests together into little chunks. This makes reading music much easier to accomplish at a quick glance.
A bar line acts the same way in music as a space does between words in a book. Now, take away those spaces and smash all the words and letters together. How easy is it to read? It may take you 2-3 times longer to read the same book.
If you don’t believe me, try and read this:
Itmaytakeyoutwiceaslongtoreadthisthaniftherewerespacesbetweenthesewords
Now, let’s put some lines between the words. Is it any easier to read?
It|may|take|you|twice|as|long|to|read|this|than|if|there|were|spaces|between|
these|words
Your eye will naturally try to group words together in an effort to make sense of what you are reading. Visually, we don’t really enjoy seeing the smashed-together words on a page.
In the case of reading words, we prefer to see the spaces in between the words rather than the lines separating the words.
Read this same sentence one last time:
It may take you twice as long to read this, than if there were spaces between these words.
Ahh, much easier… The spaces between the words are very important. They help us identify letters grouped together to form words. Now, our eyes can track a lot faster when we are reading and our comprehension significantly improves.
In the case of reading music, the notes and rests have spaces in between them already. The addition of lines, every so many counts, better defines larger groupings to act as measures.
It would be too difficult to read music with all of the notes and rests smashed together on the same page. We need some kind of organization to help us out.
A Nice End Result
The visual importance of clearly defined words in reading a book is also necessary for reading music. The bar lines in between a certain number of music notes act in the same way as spaces do between words. The notes and rests are grouped together according to a set amount of counts or beats. This continues very consistently throughout the music until the end.
For example, one measure (or the area between the bar lines) may contain three counts total. This is what it would look like. 
The grouping of notes or rests into three counts continues until there is a time signature change or the end of the piece. Cool!
Hey, do you know what the best part about bar lines is? They are used as a way of organizing music all across the world. Now, you can easily read music wherever you go!
