A 'free kill' is a term that is used to refer to *any* objective that can be taken by a team (or player) where there is **little to no risk** to the team (or player) to take it.

This includes champions, turrets and objectives. For instance, if a team won an early teamfight at bottom 4 kills to 1, and their health and damage is still good, then Dragon might be considered a 'free kill'. The same situation can happen with Baron. 

In other cases, a team may push down to 1 or 2 inhibitors and choose to return to base. On their way back out - due to the pressure provided by the super minions - they may choose to push the remaining lane - which could guarded by an outer turret. Because of that team's map control and pressure in other lanes, this outer turret is likely a 'free kill' (regardless of health) as it would be too risky for the other team to defend it.

Similarly, if a champion is overextended, underleveled, unskilled or alone, the champion may be considered a 'free kill'. ([See Justschge's answer)][1]

  [1]: http://gaming.stackexchange.com/questions/174227/what-is-a-free-kill-in-lol/174228#174228