Actionable and specific mean that the person on the receiving end has a set of concrete actions to take to implement.
For example, "get better at programming" isn't specific, and "get a PhD in Computer Science" probably isn't actionable, but "work on improving your ability to write readable code by working through the exercises in this book" is something that can be worked with (especially if you can spend some time to mentor that person!)
Kindness matters a lot, though, and it is different from niceness.
When giving feedback, some people say that they are brutally honest, but often the emphasis is on the first word, rather than the second. Things like "The module you wrote is crap because your code sucks and is hard to maintain." doesn't help in any way, shape or form.
On the other hand, others that focus on being nice often heap on the praise, while excluding or downplaying the criticism. "You worked really hard on that module, and maybe should should spend a more little time refactoring and cleaning things up" is too nice.
Kindness means sharing the good and the bad, but without being aggressive or accusatory: "Thanks for the hard work on that module. It does exactly what it needs to, but I'm really struggling to understand what's going on in there, as there are some really big methods and classes. Breaking those apart into smaller separate objects and functions would help a lot."