how to make a imagine pop out over the page

Hi i want to make an image on a wordpress site im working on pop out such as the one in this link, https://cemaesbayfest.co.uk/line-up/ when you click the big poster on there the image pops out and the background fades, how do i do this? ive tried adding an image into post on the dashboard, but when i click the image it takes ot to another window with just the image.

Any help would be appreciated