what does "on down the road "mean?
Machael Jackson sing a song named "ease on down the road ",then what does it mean? i mainly want to know some points in grammar. is "on" a prep ? or adv? and why is there another prep"down",how to know it?
"trouble is a friend "by lenka,is not bad ,and there is a sentence "So don't forget as you ease on down the road"
and do they have the same meaning ?

Question

what does "on down the road "mean?
Machael Jackson sing a song named "ease on down the road ",then                           what does it mean?  i mainly want to know  some points in grammar.   is "on" a  prep ? or adv? and why is there another prep"down",how to know it?
"trouble is a friend "by lenka,is not bad ,and there is a sentence "So don't forget as you ease on down the road" 
and do they have the same meaning ?

Answer

You misunderstood. "ON" belongs to the verb 'to ease' here, meaning: "To move or proceed with little effort." ON just denotes ongoing action in this case, like in 'to go on.' 

So, not using 'ease on' in: 

"So don't forget as you ease on down the road." 

Would make something like: 

"So don't forget as you gently go (keep on going) down the road."

Answer

Hi Bruce,  in both songs they mean the same thing, it means to drive slowly down a road, don't rush.