Cookies are a small file saved on people’s computers to help store preferences and other information that’s used on webpages that they visit. Cookies may be used by third party vendors like Google, Facebook to show ads to based on past visits to the website.
Such data can be used for remarketing and also finding similar audiences by these 3rd party vendors to show personalized advertisements. Such vendors are able to show ads to you across internet because of their wide reach and tie ups with websites and content creators.
In case you wish to opt out of personalized ads you can switch it off from the Google marketing platform opt-out page.