Hello,
Useful posts are posts which actually bring information, help users with their problems and so on. Useless posts are posts which contain no information, random video's in help for example and so on.
The green box on your profile is your Community Reputation. People can upvote your posts and if they do, you will gain a Reputation point. They actually are pretty useless.
Your post count is shown at the top of your profile, top left-hand corner. Post count can be gained by posting in the help-section and the General Discussion area. At most language subforums, post count cannot be gained.
I hope this is enough information about it. If you want to know something else, just post a new message here with the question. Oh, and welcome to the forums!
Kindest Regards,
ThatCrazyPillow
TruckersMP Moderator