FlareSolverr is a proxy server to bypass Cloudflare protection commonly applied to indexers / trackers, so that they can be auto-queried by Prowlarr.
Once you've enabled it in your account, configure it as a proxy in Prowlarr under Settings --> Indexers as follows:
flaresolverrflaresolverrhttp://flaresolverr:8191/Then add the flaresolverr tag to any indexers requiring it.