Rendering creates new video files on your system in a folder called “Premiere Pro Video Previews”. Once rendered, your timeline should play smoothly. Even if a clip is fully covering the screen, any clip beneath it on the timeline is “implicated” in the render.

Do I need to render in Premiere Pro?

There are no restrictions about your source footage, you don't need to pre-render the timeline before you export. This works even if you're doing lots of effects and color correction in Premiere.

Why do you need to render a video?

Why do you render a video? It's quite simple. You want your video project to playback in real time with little stutter and with all effects and everything else in place. When rendering anything, you may be adding effects, layers, adjustments in color, multiple audio tracks, etc.