How can commas change meaning?
My son has a T-shirt that says:
Let's eat Grandma!
Let's eat, Grandma!
Commas save lives
Or on a comedy show called Benny Hill:
What's that in the road, a head?
What's that in the road ahead?
Commas break up sentences into bits that go together. They divide phrases and group things together. So depending on where we put a comma (or not put a comma), we can change the meaning of the sentence.
In the first example, the use of the comma changes the word "Grandma" from that which will be eaten to someone who is going to eat.
In the second example, the taking away of the comma changes the word "ahead" from something that could be in the road to the location on the road that is in question (and I added a space in the word to help see the change in meaning).
Even in dealing with lists, the lack of commas can change meanings:
I like yellow, green and blue
I like yellow green and blue
So here we've changed the items in the list.