CLS is short for "cumulative layout shift". This attempts to quantify the amount of "jitter" that a viewer experiences on your site. When I say, "jitter", I'm talking about when the contents of your web page bounce around after loading. This typically happens because of things that are loaded after your page loads.
The biggest culprit in this regard is typically advertisements. These are loaded dynamically. And because they sometimes have variable heights, they can cause the layout of your contents to shift accordingly when they are loaded. Check your ad partner's settings as many now have CLS optimizations that can be enabled.
Slickstream also loads after your page, and one the components, the static recommendation Filmstrip, may contribute to CLS.
How to mitigate CLS?
The solution is to preallocate space for the filmstrip when your page first loads.
To preallocate space for the filmstrip in your page template, you can select the appropriate setting in the Slickstream Plugin or manually add the following container to your site.