Actually, those features are easy to understand, from the word itself, "reply" it is simply replying on the post, just like other features, try to use all of the features so you will be able to know what will happen, or if you really don't even know just look the thread given by @jawhead, it is the all feature of this forum and the definition of it one by one.
Asking is really good because it helps us to know something, that is what we really need if we don't even know things we want to know, but are you really serious asking those features? Just use our common sense and then we will be able to know what are those things.
Hello buddy, welcome to this forum, I am gonna tell you the things I know about those terms one by one.
Reply - it's just a way for you to reply to the specific post, if you want to say something on the thread then click reply and say whatever you want.
Notify - if you want to be notified once there would be a reply on the thread, just click it. There would be an email notification for you.
Mark unread - it's just something related to our message inbox, if we want a message new to us just click it.
Print - it's just a picture copy, try to click it and you will see what will happen.
It would be like this.
Good job, I haven't tried the print button, so I amazed when I tried to click it, from the word itself, it just prints the things on the monitor but it chance only mild things to make it organized.