"Riding the slide" usually means holding on to the slide so you control the speed it closes.
Done by fictional characters to make loading their gun "quieter". Done by some real people who think the fictional characters are right. The drawback to this is that fictional character's guns jam ONLY when its in the script.
As mentioned, riding the slide (closing it slowly) when chambering a round can result in the slide not being fully shut, when you think it is. Can, not will 100% of the time. But even once in a while it a bad thing, because if the slide isn't fully shut, when you THINK it is, that could mean no bang when you want bang!
There is a time when riding the slide is the right thing to do. That is when the gun is EMPTY!!!! That is because when the gun is empty, it hurts nothing, and might actually have some benefit. No standard service grade semi auto will be harmed by slamming shut empty. Some OTHER designs might be, and a highly tuned match gun MIGHT be.
For those guns, riding the slide EMPTY is not a bad thing. Riding the slide while loading ALWAYS is.