Having a yellow streak is being cowardly.
1. I went rock climbing, but my friend backed off. He has a mile-wide yellow streak running down his back.
**NOTE** ‘mile-wide’ simply means long. It’s just the way we use it in conversational English. A ‘streak’ is basically a long, thin mark that is different in color from its background. So you can easily visualize ‘a mile-wide yellow streak down someone’s back.’ However, you could also say: ‘I went rock climbing, but my friend backed off. He has a yellow streak for activities of this kind.’
2. The public accused the minister of having a yellow streak when she refused to answer their questions and walked out of the talk show.
‘Yellow belly’ is also used to suggest cowardice.
You can’t expect a yellow-bellied person like him to stand up against those aggressive bullies.