[bluGill]

For starters it means reading the job ad and understanding if you are even a fit for it. You can then take the various keywords in the job description and make sure those are the ones you use and not some variant that HR won't realizes is the same thing. Generally you can tell they are looking for someone to do something in particular so you can add a line or two about doing something like that, while taking away some other line they don't care about. (In the US we use resume not CV, resumes are support to be short: once you have been around for a few years there will be things you can do that just don't fit on the page)

If you are just finishing college [or worse looking for an internship] you will have trouble putting enough on a page to attract attention. Once you have been around for a few years though you should be cutting things - and that means there are things potentially relevant you can put back on.

It also depends on how focused you are. If you are only interested in OpenGL 1.1 jobs you would cut anything not related that and just have a single resume that you don't need to focus. I used openGl 1.1 to make a point: it is obsolete so normally you wouldn't put anything about it on a resume - but there is a small chance you would encounter it as a nice to have in an otherwise interesting job (If the job wants someone who knows Vulkan but is 10% maintaining old products: openGl 1.1 might catch their attention even though you have no Vulkan experience)