ELI5

I've heard of this concept multiple times throughout my time on the internet, but I never understood what causes a "server load". Now that I'm in one of the biggest instances on Lemmy, this is one of the significant issues that we face. So what is a "server load", why does it slow down websites instead of stopping them? Does the load on a server increase by the amount of information in it, the browsing of information, or both? Does upvoting and downvoting cause a load too? Does saving posts and comments to your profile also cause a server load?