Aspect ratio is the shape of your frame - widescreen 16:9, square 1:1, vertical 9:16, or cinematic 2.39:1. Getting it right for delivery, and knowing the difference between checking framing and baking it in, avoids nasty surprises in the final file.
There are two aspect ratios to think about:
Often they match. When they don't - delivering a vertical cut-down of a widescreen edit, say - you need to decide how the picture fills the new shape, using crop, letterbox, or the Reframe effect.
Viewer guides help you compose for delivery without changing the picture:
Guides are covered in full in Viewers and playback. Remember: guides only show framing, they never change it.
To actually change framing for delivery, you bake it into the picture with Reframe or the transform tools - repositioning, scaling, and blanking the image to the target shape (see Transform, crop, and reframing tools). This is what makes a vertical or square deliverable from a widescreen edit.
When the picture and the delivery frame are different shapes, you either crop the picture or pad the frame:
Reframe's blanking options handle this, letting you fit the whole image with bars, or fill the frame by zooming and cropping. Choose based on whether preserving the whole picture or filling the frame matters more.
Before you export a final deliverable:
Tip: Decide your delivery shapes before you finish editing. Framing a shot so it works in both 16:9 and 9:16 from the start is far easier than rescuing it at export.