Aspect ratio in relation to projectors refers to the proportion between the width and height of the image projected by the projector. It is usually expressed as a ratio, such as 4:3 or 16:9.
The most common aspect ratios for projectors are 4:3 and 16:9. A 4:3 aspect ratio is the traditional aspect ratio for standard-definition TVs and computer monitors. It corresponds to a width of 4 units and a height of 3 units. A 16:9 aspect ratio is the standard aspect ratio for high-definition TVs and is used for most current movies and TV shows. It corresponds to a width of 16 units and a height of 9 units.
The projector's aspect ratio must match the aspect ratio of the projected content. If the projector's aspect ratio does not match the aspect ratio of the content, the image will be distorted, and parts of the image may be cut off. Some projectors have an "aspect ratio adjustment" or "image scaling" feature that allows you to adjust the aspect ratio to match the content. It is done by changing the resolution of the image or by adding black bars to the picture.
Many modern projectors are capable of displaying non-standard aspect ratios such as 21:9 and 2.35:1. These aspect ratios are used for certain types of content, such as movies and videos that were shot in a cinematic format, or for certain types of graphics and presentations that require a specific aspect ratio, most of the mainstream projector brands have projectors with this capability, it's essential to check the projector's manual or the manufacturer's website to see if the projector can display non-standard aspect ratios and how to use it.