Create a constant and variable

Not sure exactly what you are asking or what the issue is, but this is explicit typing:

var str : String = "Hello "

and this is implicit typing:

var greeting = str + modernProgramminglanguage

What that means is that swift can figure out that greeting is a String given that you are assigning a String to it. If it were explicit it would be:

var greeting: String = str + modernProgramminglanguage

But I get the feeling that you are having a problem with a challenge. If you don't say what the challenge wants, or better, give a link to the challenge, then we're left guessing what the issue is.

P.S., re nomenclature. The standard lingo is to declare variables, e.g.,

var greeting: String  //creates the variable, type is required

and to initialize them (give them a value):

greeting = "hello"  //gives it a value

Declaration and initialization can be done in one step:

var greeting = "hello"  //creates and gives it a value in one step, type not needed as swift can infer from "hello"

Ok, I don't know which challenge you're referring to but I'm guessing you're supposed to use string interpolation. instead of adding the strings together you should do:

var greeting = "\(str) \(modernProgramminglanguage)"

string interpolation allows you to use variables inside a string instead of having to adding them together