const means you can't write to the const parameter in the body of the function. Beginners have trouble understanding CONST parameters since it could imply that you can only send a constant into the function and not a variable. That is not the case. Just think of the "const" as a read only parameter DURING the the function.

For example:

program Project1;

{$mode objfpc}{$H+}

procedure tester(const test: string);
begin
  if test = 'ya' then
    writeln('yes')
  else
    writeln('nope');
end;

var
  myvar: string;       //this is a var
begin

  myvar:= 'blah blah'; //even though this is a var...
  tester(myvar);       //you can still send it to the procedure, which will turn it
                       //into a read only constant for you now.        

  readln;              //pause program
  
end.

var means you can write to the var parameter, unlike the const parameter. This means you can modify your var in the function body. You can write and modify the var outside of the function too, as it goes IN and OUT of the function. A VAR parameter is like a INOUT parameter.

Using const parameters that are "read only" inside the DLL is a very bood practice if you are concerned about memory management issues.

  FooBar(const input: string); 

Think of var parameters like "write to" parameters that you are going to pass.

  FooBar(var input: string); 

If you don't explicitely state what your parameter is, i.e.

  FooBar(input: string); 

Then you cannot be SURE what your parameter is.

To be clear in your code, it is better to explicitely state what your parameter is when you can. Some people just skip the var and const - but I prefer being explicit and stating that I will not write to the string. Sometiems I forget to use const, but when going over code a second time, I try to put it in. The only issue with the const parameters is they make the declarations quite long:

  function test (const s: string; const s2: string); stdcall; // much longer

compared to:

  function test (s: string; s2: string); stdcall; //much shorter

Since a pchar is pointing to some memory, it will act like a reference even if you do not explicitely declare it as a var parameter. Whereas if you do not explicitely declare other types, they are usually sent in by value (not the original data being sent).