* "Let's eat, Grandma." This sentence means you are inviting Grandma to eat with you.
* "Let's eat Grandma." This sentence means you are suggesting that you eat Grandma (which is obviously not a good thing).
Here's another example:
* "I saw the man with the telescope." This means you saw a man who was holding a telescope.
* "I saw the man, with the telescope." This could mean you saw a man, and then later saw a telescope, or that you saw a man who was separate from the telescope.
Commas are used to separate parts of a sentence and to clarify meaning. Placing them incorrectly can lead to confusion and even unintended humor.