The first time your browser fetches something, an image, a link a iframe, js file, css file, whatever, usually the fetched data is stored in the browser cache (on your disk). If the browser needs the same data again within a short time it gets it from the cache rather than reading it again from the source.
So, for instance, if you have a lot of identical images on a page the image only gets read once. This reduces internet traffic, server loading and makes your pages appear faster, great stuff.
However, if say, you're using the image for tracking it's a problem because you want th fetch the image each time it's used, not just once.
So, the typical solution is to add an extra parameter that doesn't really mean anything (often just a random number) so each image url is unique and so doesn't get a cache hit.
that's a cachebuster!
BTW. the ebay link generator code has two parts the first is the link, the second the tracking image, if you don't include the image you don't get an impression.
