what is mean by sub assambly & then how can i had sub assambly into the new assambly file
Try the help function in Solidworks.
You will find the answer there on most of the questions.
But as Scott answered it is assembly in an assembly, one of the basic features in SW. This is how you build models.
Start with parts, and build (sub) assembly using mates (constraints)
Just try it, not one of the most difficult actions...