You can think of inline functions as a copy/paste instruction for the compiler. Each time the compiler sees a call to a function marked with inline, it will replace the call with the concrete function body.

It makes sense to use the inline function only if it's a higher-order function that receives a lambda as one of its arguments:

inline fun doesntMakeSense(something: String) {
    println(something)
}

This is the most common use case where you would like to use inline:

inline fun makesSense(block: () -> String) {
    println("Before")
    println(block())
    println("After")
}

You call it as usual, with the block body:

makesSense {
    "Inlining"
}

But if you view the bytecode, you'll see it's actually translated to the lines produces and not to a function call:

println("Before")
println("Inlining")
println("After")

In the actual code, you'll see the following:

String var1 = "Before"; <- Inline function call
System.out.println(var1);
var1 = "Inlining";
System.out.println(var1);
var1 = "After";
System.out.println(var1);

var1 = "Before"; // <- Usual code
System.out.println(var1);
var1 = "Inlining";
System.out.println(var1);
var1 = "After";
System.out.println(var1);

Notice there's absolutely no difference between the two blocks.

Since the inline function is copy/paste, you shouldn't use it if you have more than a few lines of code. It would be more efficient to have it as a regular function.