When editing a video, you need a digital process to export a product with a realistic image. This is the role of Rendering: an essential feature that smoothes the transitions of effects and layers over a three-dimensional space or model, so they can be perceived as natural to the human eye.
Imagine a wooden door so old and heavy that only by applying mechanical oil can it open smoothly. This metaphor, where the mechanical oil would be the rendering process, can serve to illustrate what this action does before showing you how to render in After Effects.
To see how Rendering works, you must import a minimum of a couple of Layers; so start by bringing at least two files into your Timeline.
Once you have your files on the Timeline hit V, and then, Select the layer at the top of the list. Decrease its size in the Composition Panel by holding down the corners of your file so you can see both layers on your screen as well. After that, place the scaled layer in a new position.
Keep applying some adjustments to your layers. For example, try to crop the layer you previously scaled to use only a few seconds of that file:
Now you have moved and split your layer, go to the Time Indicator and:
You've probably already noticed when you played back your video, that not only did After Effects take a while to respond. The program also did something else, like creating a new green line on the Timeline. This means After Effects is making a Preview so you can see what your clip would look like if you exported it at that moment. In other words: it's rendering your video.
Rendering your project will be faster or slower depending on the number of layers, effects, and adjustments you include in your composition. However, the green line will indicate the percentage of your project that is ready to be played smoothly.
Remember rendering your videos is helpful for you to preview the transition of your graphics and animations before exporting them.