Read the following sentence.

Jack plays the violin.

This doesn't say who Jack is, so we could add a second sentence to let readers know.

Jack plays the violin. Jack is my brother.

Although this makes sense, it sounds better to add the extra information to the first sentence.

Jack, my brother, plays the violin.

We use commas to make the sentence easier to read and understand. Without the commas it would look strange.

Jack my brother plays the violin.

The commas always go around the extra piece of information.

Mr Smith mended our washing machine. Mr Smith is our plumber.

Mr Smith, our plumber, mended our washing machine.