It's often a good idea to comment "why" the code exist, if it is non-obvious (e.g. it's obvious to sanitize input parameters). The comment must be short and possibly point to a ticket wrote somewhere else.
It may be a good idea to comment "what" the code does, if it isn't clear (the code itself is "how" it is done, but "what" does it do may be hard to read, e.g. sometimes you use a clever hack for performance reasons).
It may be a good idea to comment "what" the code does, if it isn't clear (the code itself is "how" it is done, but "what" does it do may be hard to read, e.g. sometimes you use a clever hack for performance reasons).
As always, handle with care :-)