Image resizer is a tool that allows you to change the dimensions (width and height) of an image. It can make an image smaller or larger, depending on your needs, while maintaining or altering its aspect ratio (the proportional relationship between its width and height).

Image resizers are commonly used for:

1. Reducing file size for faster loading times, particularly for websites.
2. Fitting images into specific dimensions for printing or displaying on different platforms.
3. Adjusting images for social media or other formats that require specific sizes.

Some resizers offer additional features like cropping, rotating, or compressing images along with resizing. You can find image resizers in various formats, including online tools, mobile apps, and desktop software.