An addon is an unofficial script loaded in a browser or in the standalone client, usually for quality-of-life features. Addons can change the user interface, add extra statistics, etc. This is different from modding as it only changes your client and can't add new areas or enemies or other content.
Some addons can be found here: https://gitlab.com/Isleward-Modding/addons
The most popular addon is Waddon.