The other day, when I went to a meeting for a local book club, one of the ladies in the club complained about the ads shown to her every time she went on the Internet. She had searched for a part for an unusually-shaped toilet, and now every time she went online, she was subjected to toilet ads.
The lady's problem is users of many popular search engines -- not just Google, but also many of its competitors -- keep tabs on its users and track them. What you've shown an interest in something, that's recorded and then used to target ads at you. There's a good new book out, "Dragnet Nation" by Julia Angwin, that goes into detail about the tracking, and the author's efforts to evade it.
One way to evade some of the tracking is to use a search engine that doesn't track you, such as Duck Duck Go.
Most browsers have a default setting choosing which search engine you'll automatically use. If you go into the settings, you can change which one you use. I have reset my Google Chrome browser to use DuckDuckGo. I have the DuckDuckGo app installed on my smart phone, and I usually use that instead of Google.