Hi John and welcome to the forum.
Add-on's are different from a webrep. Add-on's can be good or bad depending on what they are. Some are very helpful in a browser and protect you by adding a layer of security and in some cases privacy. An example of this is if you look at my Signature, you will see some add-on's for both my machines (you most likely have Windows, so refer to this).
However, sometimes, people put add-on's their machines without realizing it, and they tend to not add to security and may contain ad-ware or tracking. Perhaps this is what Avast detected and the
terminology used is a "poor reputation." Maybe is is a poor choice of words, but again this is a global company and the words are translated into many different languages.
I think you are familiar with webrep based on your post, so I just wanted to point out the difference that these two are not the same.
Please let us know if you have any additional questions.