As the basic documentation for Cards says, when you choose to have a card with a cover image, that image will be used as the cover of the post corresponding to the card in your site.
This means an
<img> tag it will be placed inside the
<header> of the
<article> tag representing your post, both in lists and labels views, where relevant cards are shown in a list, and in the proper card page.
In all these places the
<img> tag will be surrounded by an
<a> tag, for making it clickable like the title of the card itself.
My cover isn't showing
Sometimes a theme can make a design decision and deliberately hide cover images. That sometimes is for better, but you have the power to nullify their decision by adding the proper CSS declarations.