Bounce rate: What is it, and why should I care about it if I have a website?

I had an email exchange with a client last week about their website. Specifically, they wanted me to direct traffic to another website to complete a booking task. I explained why this is never in your website’s best interest regarding SEO (search engine optimization).

However, while explaining, I realized that if she didn’t understand Bounce Rate, she probably would not understand my explanation.