Pixel redirectors work similar to click trackers. You send a click to the pixels redirectors URL instead of the destination page. The pixel redirector collects the same data as a normal pixel and then redirects the click on to the final destination URL.
Many people were using an HTML meta refresh with a regular pixel on it to perform this function. This was not a reliable method because the pixel did not always have time to load before the redirect occurred. Using a pixel redirector solves this problem.