You can add a comment on any video which has this feature enabled; you can also enable or disable commenting on your videos from your privacy settings under the General tab.
The rules for commenting
The general rule for commenting is quite simple: be cool and play nice. You are free to respectfully agree or disagree while providing critical feedback in the comments area of Zookap, but please don’t spam, and be a person (not a robot). See our guidelines about spam and misleading or deceptive metadata for more info.