is the proportion of width to height of the content you watch. As you can see in the illustration, it changes the number of details that you see in the picture.
For TV, it is 16:9, the movie you watch at the cinemas is 2.35:1. So, decide on your favourite content to decide your aspect ratio. I'm listing down the aspect ratios for popular content formats to help you decide. A pretty safe option is go for 16:9 as all projectors and distribution formats (Blue-Ray discs, Netflix, Prime) support this universally. If you already have a projector, find this value in the specifications.
Below is a list of popular content types and their aspect ratios.