Websites can store unwanted data using all sorts of methods: besides the usual cookies, there are also the local and session storage, the IndexedDB API and more caches as well. Bisc will try to go through each of them and remove all information from websites that are not explicitly allowed (ie. a whitelist of domains). It was created for qutebrowser, but it actually supports the storage format used by Chromium-based browsers, which (sadly) means almost every one nowadays.