I recently filed a motion in juvenile court. If it gets rejected/dismissed is an explanation given?

A motion? A little vague, doncha think? If the motion is legally sound and you support it with cases and code sections you have a greater chance of being treated seriously by the judge. Judges are too courteous to tell people they are legally incredibly uninformed. It's not the judge's job to do legal research for you, or to give you a 3-year law school education in 3 minutes or less, nor is it a reasonable expectation on your part. Google "courts of record" and "courts not of record" [your state].

Ask the judge. They can CHOOSE to explain the ruling to you or not to explain it. There IS NOT a generally correct answer.

You are lucky if they tell you anything, they usually need 45 days just to figure up how to say, no you can not have paperwork. If they were employed by private industry, all court officials would be fired to incompetence, they are way too lazy to do things in a timely manner, and do all they can to not give material to the defense.

Ask the Judge to allow you to state your reason in the Order that is presented.

It depends on what the motion is, the judge may give an explanation, ask for an response from the opposition or simply make a ruling.

Law and Regulation Questions 

Laws are different depending on where you live. When asking questions about law, you need to state the applicable city, county, state, and/or country and age. If a regulation you may need to give program name. 

Please update your question (the edit button below your question) and not in a comment to my answer, so others will see it. 

In many cases people can’t begin to guess an outcome unless they know really specific details.