Image Renditions is predefined width and height to use them for images.
For example you can define a rendition call “Thumbnail” with width=70px and height=70px then you can use this in page layout to apply these dimensions on an image.
To use Image Renditions first you have to enable BLOB Cache
You can find this feature in the site settings of publishing site.
After opening the page, it will look like the following image
To use it, edit any page and insert image within the page.