why we need to put double " " between string ?

Answer 1

to specify that it is string, not integer.
these are some conventions to specify data type
int a= 5
char b = 'x'
string c = "hi raj"
like if you address ,
10 Mumbai, India
123456
you want to treat these integers as a part of address then specify it as string

Answer 2

The double quote is actually a string containing a space. Otherwise "David Williams" would be "DavidWilliams"
David + "space" + Williams = DavidspaceWilliams
David + " " + Williams = David Williams