What is advertising retargeting?

Recently, two people who live rather geek-free lives asked me the same question:

What is advertising retargeting?

I wasn’t pleased with my answer either time. I think I used the word “stalking.”

Yesterday, I saw an ad that reminded me of the question and provided me with a great example to answer with a show-and-tell.

Here goes:

Say, you have an interest in bicycling and you spend a few minutes on a Saturday morning visiting some online retailers to see what they have related to something you’re thinking about getting. Later, you notice that everywhere you go on the internet you see ads that look like the ad below.

That’s advertising retargeting.