Target of target is an UI option disabled by default.
This thing can show you who is targeting you (clicking your character) and what spell is casting. It is very useful to have it enabled in PVP because it can show who and when is attacking you and also it shows the casting bar of the enemy and this helps you to interrupt.
To enable Target of target, press escape in game > go to interface > Combat (the second in the menu) and check “target of target” and select “always”.
Another use is for RP, if you want to check who is stalking you and who is inspecting your gear.